thanks again for your support. It looks like the NUC does not like an specific HDMI port at the TV. But this is the ACR port which I have to use with the AVR were the NUC is connected to. So it looks like TV and Libreelec incompatibility as 4K 60Hz is supported at every port but not for example UHD Deep Colour (4:4:4). After connecting to the Deep Colour Port 4K 60Hz was working.