Thanks so much! Never heard about this developer mode and how to activate it.
Now I’ve activated it and tried the trick with manual USB configuration and it worked. At least for some time – it de-connected after I tried to copy some movies from the camera folder of my phone to my laptop. And I’m actually missing most of my pictures in the camera folder, but that’s probably a different problem…
In Build number it says: FP2-gms-18.04.1 release-keys