I finally got a chance to test this last night, and playback fails on my setup now also. It pulls up the show information, with the next show information listed right below it, the screen is all black, and after about 5 seconds it fails saying something about not being able to use or allocate (can't remember now, sorry).
I just went to test again, to confirm the message, but now it's working again. I don't think it is/was a signal quality issue, since I tested on several channels, and it's been working fine for years.
Oh, I think I might know what it was. Last night, WMC was recording something while I tried, but only on 1 of the tuners. That must have messed up MC's ability to use the second tuner. This used to work fine in the past, I'm not sure when this problem started.
Anyway, hopefully that helps lead to a solution
I don't know if it matters, but the HD Duet is a single input, dual tuner; meaning it splits the signal from one COAX input to the card into 2 streams.
For a $70 card, I've been very happy with it so far.