As it seems, the anker of the starter motor was not the only thing that is broken with my F650ST. The battery is OK, the starter motor running fine if it's running freely, the engine can be started by towing the bike. At least while the generator cover is removed, I can also manually put the gearwheels and all other parts in motion without too much effort.
So what am I missing? No, the brushes and the case of the starter engine don't have a short-circuit, and yes I already bridged the starter relay to make sure it's not faulty.
Now the only idea left I have is that something is mechanically blocking the starter engine, once all parts are mounted together. Will give it a (risky!) try to run it with open generator case.