Q:

marshal data too short

My application requires to keep large data objects in session. There are like 3-4 data objects each created by parsing a csv containing 150 X 20 cells having strings of 3-4 characters.
My application shows this error- "marshal data too short".
I tried this- 

Deleting the old session table.  
Deleting the old migration for session table. 
Creating a new migration using rake db:sessions:create.  
editing the migration manually, changing text: data to longtext: data. 
running the migration using rake db:migrate.

other details of application--
In view "index.html.erb"- There is a link that makes ajax call to an action in controller, that action parses large csv file and makes an object out of it. this object is stored in session.
ERROR LOG
` ArgumentError in Scoring#index

Showing app/views/scoring/index.html.erb where line #4 raised:

marshal data too short

Extracted source (around line #4):

1: 
2: <form id="myForm">
3: 
4:   <%= link_to_remote "get csv file",
5:        :url      => { :action => 'show_static_1' },
6:        :update   => "static_score",
7:        :complete => "$('static_score').update(request.responseText)" %>

Application Trace | Framework Trace | Full Trace

/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/session_store.rb:71:in `load'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/session_store.rb:71:in `unmarshal'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/session_store.rb:110:in `data'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/session_store.rb:292:in `get_session'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/base.rb:1448:in `silence'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/session_store.rb:288:in `get_session'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/session/abstract_store.rb:168:in `load_session'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/session/abstract_store.rb:62:in `send'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/session/abstract_store.rb:62:in `load!'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/session/abstract_store.rb:70:in `stale_session_check!'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/session/abstract_store.rb:61:in `load!'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/session/abstract_store.rb:28:in `[]'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/request_forgery_protection.rb:106:in `form_authenticity_token'
(eval):2:in `send'
(eval):2:in `form_authenticity_token'
app/views/scoring/index.html.erb:4:in `_run_erb_app47views47scoring47index46html46erb'

/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/session_store.rb:71:in `load'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/session_store.rb:71:in `unmarshal'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/session_store.rb:110:in `data'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/session_store.rb:292:in `get_session'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/base.rb:1448:in `silence'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/session_store.rb:288:in `get_session'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/session/abstract_store.rb:168:in `load_session'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/session/abstract_store.rb:62:in `send'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/session/abstract_store.rb:62:in `load!'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/session/abstract_store.rb:70:in `stale_session_check!'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/session/abstract_store.rb:61:in `load!'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/session/abstract_store.rb:28:in `[]'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/request_forgery_protection.rb:106:in `form_authenticity_token'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_view/helpers/prototype_helper.rb:1065:in `options_for_ajax'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_view/helpers/prototype_helper.rb:449:in `remote_function'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_view/helpers/prototype_helper.rb:256:in `link_to_remote'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_view/renderable.rb:34:in `send'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_view/renderable.rb:34:in `render'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_view/base.rb:306:in `with_template'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_view/renderable.rb:30:in `render'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_view/template.rb:205:in `render_template'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_view/base.rb:265:in `render'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_view/base.rb:348:in `_render_with_layout'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_view/base.rb:262:in `render'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/base.rb:1250:in `render_for_file'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/base.rb:945:in `render_without_benchmark'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/benchmarking.rb:51:in `render'
/usr/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/core_ext/benchmark.rb:17:in `ms'
/usr/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/core_ext/benchmark.rb:10:in `realtime'
/usr/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/core_ext/benchmark.rb:17:in `ms'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/benchmarking.rb:51:in `render'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/base.rb:1326:in `default_render'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/base.rb:1338:in `perform_action_without_filters'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/filters.rb:617:in `call_filters'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/filters.rb:610:in `perform_action_without_benchmark'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/benchmarking.rb:68:in `perform_action_without_rescue'
/usr/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/core_ext/benchmark.rb:17:in `ms'
/usr/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/core_ext/benchmark.rb:10:in `realtime'
/usr/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/core_ext/benchmark.rb:17:in `ms'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/benchmarking.rb:68:in `perform_action_without_rescue'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/rescue.rb:160:in `perform_action_without_flash'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/flash.rb:146:in `perform_action'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/base.rb:532:in `send'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/base.rb:532:in `process_without_filters'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/filters.rb:606:in `process'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/base.rb:391:in `process'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/base.rb:386:in `call'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/routing/route_set.rb:437:in `call'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/dispatcher.rb:87:in `dispatch'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/dispatcher.rb:121:in `_call'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/dispatcher.rb:130:in `build_middleware_stack'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/string_coercion.rb:25:in `call'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/string_coercion.rb:25:in `call'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/head.rb:9:in `call'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/methodoverride.rb:24:in `call'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/params_parser.rb:15:in `call'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/session/abstract_store.rb:122:in `call'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/query_cache.rb:29:in `call'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/connection_adapters/abstract/query_cache.rb:34:in `cache'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/query_cache.rb:9:in `cache'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/query_cache.rb:28:in `call'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/connection_adapters/abstract/connection_pool.rb:361:in `call'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/failsafe.rb:26:in `call'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/lock.rb:11:in `call'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/lock.rb:11:in `synchronize'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/lock.rb:11:in `call'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/dispatcher.rb:114:in `call'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/reloader.rb:34:in `run'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/dispatcher.rb:108:in `call'
/usr/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/rails/rack/static.rb:31:in `call'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/urlmap.rb:46:in `call'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/urlmap.rb:40:in `each'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/urlmap.rb:40:in `call'
/usr/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/rails/rack/log_tailer.rb:17:in `call'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/content_length.rb:13:in `call'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/chunked.rb:15:in `call'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/handler/mongrel.rb:64:in `process'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:159:in `process_client'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:158:in `each'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:158:in `process_client'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:285:in `run'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:285:in `initialize'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:285:in `new'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:285:in `run'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:268:in `initialize'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:268:in `new'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:268:in `run'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/handler/mongrel.rb:34:in `run'
/usr/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/commands/server.rb:111
/usr/local/lib/site_ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
/usr/local/lib/site_ruby/1.8/rubygems/custom_require.rb:31:in `require'
script/server:3

/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/session_store.rb:71:in `load'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/session_store.rb:71:in `unmarshal'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/session_store.rb:110:in `data'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/session_store.rb:292:in `get_session'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/base.rb:1448:in `silence'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/session_store.rb:288:in `get_session'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/session/abstract_store.rb:168:in `load_session'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/session/abstract_store.rb:62:in `send'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/session/abstract_store.rb:62:in `load!'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/session/abstract_store.rb:70:in `stale_session_check!'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/session/abstract_store.rb:61:in `load!'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/session/abstract_store.rb:28:in `[]'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/request_forgery_protection.rb:106:in `form_authenticity_token'
(eval):2:in `send'
(eval):2:in `form_authenticity_token'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_view/helpers/prototype_helper.rb:1065:in `options_for_ajax'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_view/helpers/prototype_helper.rb:449:in `remote_function'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_view/helpers/prototype_helper.rb:256:in `link_to_remote'
/app/views/scoring/index.html.erb:4:in `_run_erb_app47views47scoring47index46html46erb'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_view/renderable.rb:34:in `send'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_view/renderable.rb:34:in `render'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_view/base.rb:306:in `with_template'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_view/renderable.rb:30:in `render'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_view/template.rb:205:in `render_template'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_view/base.rb:265:in `render'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_view/base.rb:348:in `_render_with_layout'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_view/base.rb:262:in `render'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/base.rb:1250:in `render_for_file'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/base.rb:945:in `render_without_benchmark'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/benchmarking.rb:51:in `render'
/usr/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/core_ext/benchmark.rb:17:in `ms'
/usr/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/core_ext/benchmark.rb:10:in `realtime'
/usr/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/core_ext/benchmark.rb:17:in `ms'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/benchmarking.rb:51:in `render'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/base.rb:1326:in `default_render'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/base.rb:1338:in `perform_action_without_filters'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/filters.rb:617:in `call_filters'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/filters.rb:610:in `perform_action_without_benchmark'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/benchmarking.rb:68:in `perform_action_without_rescue'
/usr/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/core_ext/benchmark.rb:17:in `ms'
/usr/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/core_ext/benchmark.rb:10:in `realtime'
/usr/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/core_ext/benchmark.rb:17:in `ms'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/benchmarking.rb:68:in `perform_action_without_rescue'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/rescue.rb:160:in `perform_action_without_flash'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/flash.rb:146:in `perform_action'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/base.rb:532:in `send'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/base.rb:532:in `process_without_filters'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/filters.rb:606:in `process'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/base.rb:391:in `process'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/base.rb:386:in `call'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/routing/route_set.rb:437:in `call'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/dispatcher.rb:87:in `dispatch'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/dispatcher.rb:121:in `_call'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/dispatcher.rb:130:in `build_middleware_stack'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/string_coercion.rb:25:in `call'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/string_coercion.rb:25:in `call'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/head.rb:9:in `call'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/methodoverride.rb:24:in `call'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/params_parser.rb:15:in `call'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/session/abstract_store.rb:122:in `call'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/query_cache.rb:29:in `call'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/connection_adapters/abstract/query_cache.rb:34:in `cache'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/query_cache.rb:9:in `cache'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/query_cache.rb:28:in `call'
/usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/connection_adapters/abstract/connection_pool.rb:361:in `call'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/failsafe.rb:26:in `call'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/lock.rb:11:in `call'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/lock.rb:11:in `synchronize'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/lock.rb:11:in `call'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/dispatcher.rb:114:in `call'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/reloader.rb:34:in `run'
/usr/lib/ruby/gems/1.8/gems/actionpack-2.3.5/lib/action_controller/dispatcher.rb:108:in `call'
/usr/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/rails/rack/static.rb:31:in `call'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/urlmap.rb:46:in `call'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/urlmap.rb:40:in `each'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/urlmap.rb:40:in `call'
/usr/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/rails/rack/log_tailer.rb:17:in `call'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/content_length.rb:13:in `call'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/chunked.rb:15:in `call'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/handler/mongrel.rb:64:in `process'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:159:in `process_client'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:158:in `each'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:158:in `process_client'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:285:in `run'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:285:in `initialize'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:285:in `new'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:285:in `run'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:268:in `initialize'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:268:in `new'
/usr/lib/ruby/gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:268:in `run'
/usr/lib/ruby/gems/1.8/gems/rack-1.0.1/lib/rack/handler/mongrel.rb:34:in `run'
/usr/lib/ruby/gems/1.8/gems/rails-2.3.5/lib/commands/server.rb:111
/usr/local/lib/site_ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require'
/usr/local/lib/site_ruby/1.8/rubygems/custom_require.rb:31:in `require'
script/server:3

Request
Parameters:
None
Show session dump

Response
Headers:
{"Content-Type"=>"text/html",
 "Cache-Control"=>"no-cache"}

`

A:

I met the exactly same problem. 
My Rails version is 4.1.4. 
I solved this problem by running:
rake tmp:clear                                      

Clear session, cache, and socket files from tmp/ (narrow w/ tmp:sessions:clear, tmp:cache:clear, tmp:sockets:clear)

A:

Instead of the manual ALTER TABLE you could also do a migration with a limit, like this:
class FixSessionStore < ActiveRecord::Migration
  def self.up
    change_column :sessions, :data, :text, :limit => 4.megabytes
  end

  def self.down
  end
end

This will turn the column into a MEDIUMTEXT and also show up correctly in your schema.rb.

A:

In Rails 4 development mode I had the same error and the solution was the following: 
Delete all cache file from /tmp/cache/assets folder.
(Unfortunately rake assets:clean just haven't solved this problem.)

