I have recently experienced this problems with 7MC (RC), essentially I could not watch live tv with any of my cards in my HTPC, although playing existing media was fine (but not Recorded TV). I had to change the file permissions for the Recorded TV folder to get my old recorded stuff to play. The error pointed to a card conflict more than anything else.
The problem started to occur when i was messing around with Homegroup settings (joining, leaving, etc) with a Win7 RTM machine. I attempted to roll back my machine to an earlier restore point and play with all sorts of things, but these did not help. I eventually tried moving my Recorded TV folder in 7MC and it worked, so i then moved it back and it still worked. It would appear that 7MC did not have permission to write to the Recorded TV folder.
So, turns out that if you mess to much with Homegroup you may mess up the standard file permissions in 7MC and you will end up getting a confusing error message that is related, but not closely, to the problem.
Hope this helps anybody who stumbles upon the same problem.