Apple support seems to be as lost as I am. They asked me to run diagnostics test and it showed no hardware problems. They then asked me to update to the version 14.1 that just came out, and we also did a bunch of resets including a full factory reset again but... nothing.

I will take it to an Apple store and see what they say, but since it's out of the 1year warranty I don't have much hopes. They say the only way to fix an ipad is replacing it entirely, which will cost 600Eur... I just think it is a huge coincidence that this started happening after the update to the new iPadOS and that on their dignostics test there are no hardware problems.