Unfortunately, it appears that the recently released GitLab v15.2 appliance (which provides GitLab via Omnibus installer) has a pretty serious bug where initial login gives a 500 error... 😢
I intend to re-release ASAP. In the meantime, to workaround the issue, log in via SSH and run the following commands:
mv /etc/gitlab/gitlab-secrets.json /etc/gitlab/gitlab-secrets.json.bak
gitlab-rails runner -e production "ApplicationSetting.current.reset_runners_registration_token!"
/usr/lib/inithooks/bin/gitlab.py
And (re)set your password, email and domain.
Everything should now work...