Thanks for your help 🙂
I was not able to get MMM-PIR-Sensor running, even with your steps listed above. As it is deprecated I did what @karsten13 suggested and used another PIR (MMM-PIR). It had the same issue first, but using magicmirror-rebuild from https://forum.magicmirror.builders/topic/16074/electron-rebuild-and-magicmirror-v2-18-and-more?page=1 I was able to got the PIR working again.
So again: Thanks for trying to help and your suggestions!