I have some good news. After reading this manual (Howto connect the Nuki Opener to unknown intercom systems) I tried a minimal wiring like nello did. Which means I connected only two cables (red and black without using the clamp). I connected the red cable directly to Ta together with the cable that was already in there (BUS+) and removed the yellow and the green one. Left the black cable (BUS-) into Tb and now it works. As far as I understand it, with this setup it is not possible to use the doorbell suppression feature but at least the basic nello features are there!
Hope this helps others…
Hopefully the nuki team is fixing the bus communication so that we can use all features…