Unstable HDMI signal

[MichaelRu]

Thanks for your tips!

I made some more tests.
The same HDMI cable works between Pi and monitor.
I tried a different HDMI cable to the AVR with the same results.

With 720p in the Kodi GUI the problem comes later, but it does arise nonetheless.

I also switched input connections on the AVR. The Bluray player works everywhere, the Pi nowhere. When I connect the Pi directly to the beamer, then the display works, but I'm missing the sound of course. So it seems to be a problem specifically between the Raspberry Pi and my Yamaha RX-V475.