thanks for your help and feedback. I have completely wiped my system and started again from scratch. This time, I can add modules without any errors. This indicates, that there likely was a problem with the framework nodejs and or MM2 depended on.
There is one noticeable difference: When I am connected using SSH, I can only start the Mirror usingnode serveronly
The command I used up to nowDISPLAY=:0 npm start
does not work any more.
Is this intended behaviour?