@scooke Thanks a lot! The problem is solved.
To be specific, if you have installed Taiga with default settings, then it is configured for LDAP use, which means it also uses the Cloudron user-management.
Ok, thanks; I did not know that.
If you have invited people, who also have an account on your Cloudron (same username/email) then they might clash.
I contacted the user to verify this and will get back to you once I have his response (also with the logs as well if needed).
Related to a part of my original issue report:
Then I tried to log in to my account using a different browser and even the same browser in incognito mode but I could not log in anymore. Note that I was (as I still am) logged in to the Taiga instance in the browser which I used for my first log in (Edge), and I can still use it from there.
This is not an issue anymore with my account. I'm not sure what happened but I can log in now from any browser.
Changing root account email in fresh Discourse install
Users cannot login after password reset or using a different browser than for the registration