Thanks for the explanation. I just got the motherboard out to solder the joints like DistantStar001 suggested. Added some fresh solder too. That did not help unfortunately. So what's left is to look at the 6522 chips. Mines are soldered on. And those are way above my skillset. Too small. I will try to find someone on a local marketplace if it can be done for a reasonable price. Maybe somebody here is from the Netherlands and knows somebody?