I discovered a strange bug:

I have two webcams and these cams are both mirrored at the source

Then I create a image as new input.
On this input I place the two webcams as multiview on Layer 1 and Layer 2
When I set the two cams horizontally next to each other it is working perfectly, but when I place them trapezoidal side by side I get no video from the cameras. I see only the image.