Thank you very much for your support!
The attached action doesn't seem to have any influence on the object (I added it as the start state in a new fsm on the rigid body object) but the velocity doesn't clamp.
I changed "public override void OnFixedUpdate()" to "public override void OnUpdate()" and this works now. But I'm not sure if this is a good way to get things done because the unity documentation says that FixedUpdate should be used with rigid bodies, as you did it in your script.
I'm a bit confused why it doesn't work with FixedUpdate
Best regards,
Daniel
By the way: The script I quoted in my initial post works and it's using FixedUpdate too. Quite confusing...