Hi @Martin-Kudláček,
thank you for your input. The problem were the internal pull-ups, set to low. Usually the python library, I'm using, does this job, but there seems to be a problem with the Raspberry Pi 4. Nevertheless I was able, to get everything up and running again, by setting needed GPIO pull-ups to high via /boot/config.txt. It's not the best solution, because it should be fixed software-wise. But it's the proof, that the problem is not related to hardware. Thank you so much for helping me, figure this out.
Dirk