I’ve just noticed that there was still some activity in this thread. So thanks for your answers.
Yes, I’ve also noticed that it sometimes helps to restart cmc.
But the actual issue was – as I tried to describe above – that the extraction of my zip file destroyed the directory structure in local/lib because the link local/lib/check_mk was replaced by an actual folder so data in the link target local/lib/python3/cmk remained unchanged.
I fixed that by replacing the folder local/lib/check_mk by local/lib/python3/cmk in my zip file.