Q:

Migrating Gitlab from CE 7.14 to 8.17.8 causes unicorn to be unable to start and 502 error

After migrating from 7.1 to 7.14 successfully, I tried migrating to version 8.17.8. Unfortunately it didn't work, as now I'm getting the following error:
==> /var/log/gitlab/unicorn/unicorn_stderr.log <==
NameError: uninitialized constant UsersGroup::Notifiable
/opt/gitlab/embedded/service/gitlab-rails/app/models/users_group.rb:15:in `<class:UsersGroup>'

==> /var/log/gitlab/sidekiq/current <==
2018-05-25_06:21:50.98012 uninitialized constant UsersGroup::Notifiable

I tried running reconfigure, restart, even rebooting the server, but nothing works.

A:

If sudo gitlab-ctl reconfigure is not enough to fix the issue, then you might consider doing the same migration again, but this time minor version by minor version.
That is: follow the "Upgrade recommendations" (which you did):

We recommend that you first upgrade to the latest available minor version within your major version. By doing this, you can address any deprecation messages that could possibly change behaviour in the next major release.

But this time, try first 7.14 to 8.0.x, then 8.17.8, meaning one intermediate steps in the lower version of 8.x

