No 4k output with Coreelec

Hallo from Germany,

i m running a vim3 fanless in original case hooked up to a yamaha av receiver and it working perfectly since about 300 day.
Today a tried to set output in corelec to 3840 x 2160 end there is no output.
I have changed the hdmi-cable to the one i use succesfully with my Amazon firetv Cube and there is no difference - no 4k.
When playing youtube videos in 4k it runs in hd resolution even it is shown that the stream is beeing deliverd in 4k.

Any idea anybody?
Looking forward to hearing … :slight_smile: