Try disabling Simplink-CEC. I had the same problem and that was the only work-around I have been able to come up with. Just happened out of the blue.ARC via HDMI all of sudden not working, any fix for it, I checked all setting on AVR and TV both. any suggestion how to fix it.
I am on latest software on C6.
The TV remote no longer controls volume on my Denon AVR which is a minor inconvenience, but whaddya gonna do?