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