Hello all together,
I actually tested Greg's idea and changed the timeout of the watchdog from 0.5 to 3.1 (max possible).
The problem is solved

I am glad that it is working now!
Greg thank you very much for the detailed help. I only wished the so callled german support would act like you did.

I am sorry that they were not able to help. I really don't expect them to know the inner details of the SmoothStepper. So please don't be hard on them! In fact, it took me a few days until it hit me what it was.
Greg