Okay, now its working... i really tried everything but Clark had finally the right idea!
There is a function to switch gears for some tanks, when powering on the board is set to neutral gear.
And while in neutral the throttle is not working.
In the special case with a differential gearbox the throttle switches to a different channel so the throttle works but not the steering.
So it was just a configuration issue, which is very well hidden in the guide.

Problem solved