first of all thanks for your advice.
Nothing worked for me, unfortunately.
My last try is to deinstall my graphic card driver or try different driver settings (AMD). Maybe my card is too old. but on the other hand why does it cause disconnections?
I hope that google provide a proper solution on this at some stage.
So the chrome cast ultra is a modern PC? Faster than my eight core / 16 GB ? I dont think so 😄
If the codec is the reason than i would expect i slightly different picture quality between these codecs (thats what others say) and no issues with the connection (At least thats what stadia says in chrome)