HDMI output unreliable on Nucleus

I think that you also need to give some more information about what the other equipment is in your setup, e.g. what the HDMI outputs on the Denon are connected to. And have there been any changes recently.

I say this because my ROCK/NUC worked faultlessly with my Denon AVR when the latter was connected to a Panasonic TV. When we replaced this with a Samsung TV, the HDMI handshaking across the NUC → Denon → TV broke on a regular basis. I had to invest in a Doctor HDMI device between the NUC and the Denon to fix the handshaking problems.