To track the source of the error, you need to check production.log.

On my Omnibus edition, it was located at /var/log/gitlab/gitlab-rails/production.log.

# find / -iname 'production.log'
/var/log/gitlab/gitlab-rails/production.log

Then read the log to find the recent error:
# tac /var/log/gitlab/gitlab-rails/production.log | less

My error was:

  app/controllers/sessions_controller.rb:25:in `new'
  app/views/layouts/devise.html.haml:18
    21:                 Login to manage your projects
    20:               %p
    19:
    18:                A collection of open source projects
    17:               %h3
    16:
    15:               Droidzone Opensource project repository
ActionView::Template::Error (Inconsistent indentation: 15 spaces used for indentation, but the rest of the document was indented using 2 spaces.):
Completed 500 Internal Server Error in 353ms

Fixing it was easy. I just needed a restore a copy of the original file.
Restart gitlab after fixing the error:

# gitlab-ctl restart