I’ve finally found parts of the issue:
I have two monitors, one of which is rotated by 90°. I’ve created a new user which can log in without issues. However, when I adjust the rotation to be “correct”, I get logged out immediately.
Upon logging in again, the rotation is not saved.
Has anyone any idea how this could be debugged or solved?
How to discriminate if that’s a cinnamon or a xorg issue?
EDIT: @Gene describes something similar here though I don’t understand how he solved it…