After running the great update tutorials on gitlab.org the dashboard was totally messed up, a big „toggle navigation“ button appeared on the top and everything was misplaced and some files from the „assets“ folder couldn’t be loaded (404). As I remembered having problems with the assets before because the timeout on my Apache was too short, I reran the asset precompilations:
sudo -u git -H bundle exec rake assets:clean RAILS_ENV=production
sudo -u git -H bundle exec rake assets:precompile RAILS_ENV=production
Run this with sudo. Do a shift-reload in your browser and voilà it should work.