No System Console when logging in with GitLab

Hello there!

I solved this by editing the users table in the mattermost_production database; A query like this:

UPDATE users SET roles = "admin system_admin" where username = "your_username";

To access the mattermost database in the omnibus installation:
sudo -u gitlab-psql /opt/gitlab/embedded/bin/psql -h /var/opt/gitlab/postgresql -d mattermost_production

Cheers!