After some googeling and trial/error I got the Opener working flawlessly with the Comelit 2708W
The cabeling is as follows (in order from top):
CFP- (ignore) are for the door-bell next to your Apartmentdoor (Etagenruf)
L+ (Orange+Green) the bus cables
Clamp Yellow+Red together
In the Nuki App I selected:
Model: Bus (Urmet BIBUS) → the last one
I configured the Nuki Opener with the cable hanging out - when you do this, make sure to connect the headset - otherwise it won’t ring! (The ringer is in the headpiece, not the main board)
Only (minor) issue I had, was when the App was telling me to “press the opener” I had to press multiple times times until it worked (actually I had to restart the setup process 2-3x until it recognized the opening signal).
Once this step was over and it asked me do go to the front door, everything worked perfectly fine.