Thank you Ted,
It was the Omnicam by the HHI in Berlin, which they use for several years and projects now. Seems that they did not have that issue before. It contains 10 cams and 10 mirrors.
I ill investigate which cameras are built in.
I did render different parts to see if there would be some flickering, as that was the only reason we could think of. There was no flickering at all.
Any other idea?