I successfully performed an upgrade from 17.3 to 18 last week on my 2012 MacBook Air and it’s been performing well until this morning.
The System Updates kicked in and installed some stuff (I wasn’t paying attention) and I continued watching YouTube.
After I closed the lid and reopened it some time later I was greeted with a password prompt - nothing strange so far - but it continuously rejected my login attempts without errors.
I rebooted and the login loop continues.
Do I have to reinstall or is there some Terminal trickery I can perform?
Hopefully, the reinstall, fixed your installation. There has been some troubles with updates lately, that are causing issues. The Zorin team, recently made a post, about what they fixed.