ksteinke;

I have seen enough reports similar to yours regarding Dynamic shaft drive problems so that I suspect some design or manufacturing problems. IIRC another BF member reported problems using one in San Francisco. Again a heavy rider who reported riding out of the saddle at high power on hills. Do a search for Dynamic bikes here on BF and you will pick up multiple threads.

Try regreasing as it should not hurt but I suspect possible end play or other adjustment problems with the shaft may have developed.

As you are in Switzerland you might have done a German Ebay search for a Fendt Cardano bike. These were a German shaft drive that was supposed to be quite reliable I believe.

add grease- to Ksteinke
 

Yes, add grease. Whenever the drivetrain makes noise, add grease. Then add some more. I have more than 3000 miles, and I have filled the gearbox up with grease - no noise in hundreds of miles.
Grease is the weak spot of the dynamic bike drive. At purchase, the buyer should fill the gearbox up with grease. This will save trouble later.
MRiley