Moodle in English

Good luck - come back if that doesn't work.  That error about the array - almost looks as if a field might be messing it up - did you by chance add a new field to your student accounts?  Or is it possible that when you added the second domain to the allow list it was not in the right format?

Well, something must have changed somewhere - either on Microsoft or Moodle - no upgrades?
Have you tried with full debugging on or is the message above what you get with that? Have you looked at the two lines specified? Any help there? Have you tried setting up a separate login specifically for the student domain?

I would guess to check your meta link enrollment and make sure it is pointing towards the correct courses...

Have you put in a bug report on their github page?

There is not a grey period. Now that it is working, will be hard to know what happened. Could have been a session issue, browser cache, editing not turned on...anything really...