I don't have the DVR that you have. But I have the HDMI going from the STB to my tv. From the TV to the receiver I use the optical/digital cable. Not all TV's have an optical out.
Not sure of all the fixes you have tried...
Give this a try Main Menu > Customer Support > Self Diagnostics...
I don't have an answer for you, but I am just posting to tell you you're not alone. The digital optical output does not work on my DVR, either. I've verified that it's not the fault of my cable or my receiver. The good news for me was, I got the coaxial digital audio port to work. Good luck.
