[Jemus]

@groofster
Well I checked the adjustment of the framerate:
RPi2 --> Denon x2200w --> HDMI 1.4 TV: works
Shield --> Denon x2200w --> HDMI 1.4 TV: doesn't work
Shield --> HDMI 1.4 TV: works
Shield --> Yamaha RX-v677(no HDCP2.2) --> HDMI 1.4 TV: works

So it seems to be the combination of the Shield and the Denon AVR, I guess. Unfortunately exactly the combination of devices I d'like to use
Hope there might be a software solution like forcing the Shield TV to put out the older HDCP version but I guess it's Impossible.
So I will take a look at those HDMI splitters.