Q:

pgAdmin4 error "Failed to launch the application server, server thread exiting."

I'm getting this error on startup of pgAdmin4 on Kubuntu 18.04:

An error occurred initialising the application server:
  Failed to launch the application server, server thread exiting.

In the following window ("Configuration") I have these default values:

Python Path:
  /usr/pgadmin4/lib/python2.7;/usr/pgadmin4/lib/python2.7/site-packages
  Application Path:
  /usr/pgadmin4/web

Since I don't have a /usr/pgadmin4/ directory, all of them are wrong.
See https://stackoverflow.com/questions/43968302/failed-to-launch-application-server-server-thread-existing-on-windows for the same error on Windows

A:

For me, when I run pgadmin like Thomas Landauer says, I got this error:
$ sudo python3 /usr/share/pgadmin4/web/pgAdmin4.py
Traceback (most recent call last):
  File "/usr/share/pgadmin4/web/pgAdmin4.py", line 58, in <module>
    exec(open(file_quote(setupfile), 'r').read())
  File "<string>", line 47, in <module>
  File "/usr/share/pgadmin4/web/pgadmin/setup/data_directory.py", line 29, in create_app_data_directory
    _create_directory_if_not_exists(config.SESSION_DB_PATH)
  File "/usr/share/pgadmin4/web/pgadmin/setup/data_directory.py", line 15, in _create_directory_if_not_exists
    os.mkdir(_path)
FileNotFoundError: [Errno 2] No such file or directory: '/var/cache/pgadmin/sessions'

The error says what is the problem, so I created the folders:
$ sudo mkdir -p /var/cache/pgadmin/sessions

And after restarting pgAdmin4, it first asked for user creation, then worked.

A:

The problem seems to be that I have python2 and python3 installed. Solution (workaround) that works for me:

Locate your pgAdmin4.py. In my case, it's /usr/share/pgadmin4/web/pgAdmin4.py
Start it in a terminal like this: sudo python3 /usr/share/pgadmin4/web/pgAdmin4.py

If you have a better solution, please let us know :-)
I opened a ticket for this: https://redmine.postgresql.org/issues/3737

