Hallo Jan,
thanks for the fast response - I have 24GB of VRAM, so don't worry, there is still room to the top ;-)
In the meantime, I found the transgressor:
Obviously, it seems to be the "X-Assign - Plugin" which I used to configure my TQ.
Since I have another TQ now, I don't need this plugin anyway, so the deactivation seems to solve the problem ;-)
Thanks again and best regards
Alex