problem was: my logitech c270 webcam had stripes wandering around, I think you could call it flicker. I tried everything but nothing helped. Just changing the resolution helped a bit.
now the real solution and the real problem: adjust via v4l2-ctl -c powerline_frequency the powerline frequency. The flickering immediately stopped for me. To see which values it can have use v4l2-ctl -L
for me worked:
v4l2-ctl -c powerline_frequency=1
It was simply the power supply!!! It has an other frequency in Germany than in the home country of logitech. The technicians saw this problem coming so include such an option but… they were too silent (should have dropped an entry in the FAQ explaining that).
Ok, I use linux and the tool of logitech would surely have adjusted this but I’m windows free.