Hey! Thanks to you guys and the other linked posts, I found and fixed the same issue. I had some old overrides saved and sure enough, my Multitaps were "auto". Changed to "disabled" and all is well again in PSX land!
I have now used another sd card with a fresh new image. Did all updates as soon as I could, before configuring any inputs. It took about 3 hours and finished with some warnings on emulationstation. But nothing wild.
I have then put a psx rom onto the pi and configured one ps1 controller via usb adapter and one ps4 controller via bluetooth.
Everithing seems to work fine right now. I am going to add the n64 controllers and some more cores. But for now: Thank you very much @mitu for your support.