Mattermost: Cannot log in (or something)

I installed Mattermost in a 7.x version and it did not work as expected (authentication failed and could not set up OAuth manually afterwards).

After upgrading to 8.1.2 I am now again stuck with a Mattermost install which

• Cannot be removed, removing the settings from gitlab.rb does not seem to uninstall databases and OAuth settings
• Cannot be automatically re-authed, re-enabling the settings does not run the auth procedure
• Cannot be manually re-authed, setting values in gitlab.rb does not change the oauth settings in config.json (see GH issue 9764)
• After manually setting config.json I still only see a create team screen and when trying to create one I get We received an unexpected status code from the server (200)

There is absolutely nothing in the mattermost logs.