Thanks for the pointer!
USB debugging was disabled.
I also disabled the developer options altogether, that didn’t change anything yet.
But when I enabled USB developer options & USB debugging again (while the phone was connected), the phone suddenly recognized the USB data connection! And after deactivating USB debugging, the data connection stayed on… so seems it was some kind of software hitch!
TL;DR: Activate/Deactivate USB debugging mode for fixing this issue!