Hi Niall,
thanks a lot for taking the time to answer me with details.
The Vero 4k is connected to YSP-2700 HDMI 1 (IN 1) and advanced mode was enabled to allow HDR.
The problem is now solved.
I switched my 4k HDMI cable (certified) which was originally used on TV (HDMI ARC) to YSP-2700 OUT/ARC with a standard arc capable HDMI Cable.
I used my 4K cable with the Vero 4K without changing anything else i had in my previous setup.
everything is now normal.
Maybe the certified 4K cable has a better shield, who knows?
This problem also occured when i was using the stock cable (the one included with the Vero 4K)
Thanks a lot to everybody participating in this thread