I wonder if the incompatibility was due to something like HDCP, and needing new keys or something similar. The Xbox is fairly strict on enforcing DRM, and that might be why it wouldn't output to the TV after a while.

I recall that when I was doing console gamedev stuff, some monitors would not work if you set the console devkits into 'retail' mode, especially some of the old cheap LCDs that IT had stashed away to use as temporary loaners if someone's monitor was broken.