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.