I'm inclined to think this semms to be a common problem with the camera...
Here's a comparison video betwenn the VG900 and the Alpha7s.https://www.youtube.com/watch?v=3_PPSIlKUJE&t=218s
@2:30 there's a split-screen that looks especially ugly.
I'm in touch with Sony.
Let's see, what I can find out.
Here are two shots with the exact same settings, same lens etc.
0005.mts in full frame mode, 0006.mts in APS-C.
The latter displays the weird lines, the former seems to be much clearer.
I tried everything, I could think of:
24p, 25p, 50i and 50p. The codec seems to be h246 in an AVCHD-container. There seems to be no other option with this camera.
Also I tried different bitrates.
But I don't think any of these is the culprid since I got good results in full-frame-mode using the exact same settings as before.
Thanks for your answer.