Hey Beppe, thanks for your reply
after reviewing the problem and trying out your suggestion, the problem persists.
any other solutions i could try?

I rendered a Car with a specular Material on the windows, i tried out using all the options concerning alpha channel (straight alpha, fake shadows, thin walls, keep environment) and everything else i would somehow link to the problem but so far no luck. If i place an object behind the car in C4D ...

I had the same problem using PT and the C4D Stage object. The first frame rendered and finished fine but from there on the renders stuck at 100% and wont save the passes. The clock kept running and nothing happened. Turning off the stage object solved the problem.

Even if this is a really old topic: I will write this down, maybe it helps solving the issue for anyone else:
So i was rendering some frames of a quite heavy scene using the stage object, in which the stage Object defines the camera used for each frame. The first frame always came out fine, whereas ...