Can't play videos from iTunes store after switching machines

Symptom: my iTunes Library lives on an external drive. Recently I needed to send my primary MacBook Pro in for repair, so I authorized my backup machine successfully and used it in the interim. Upon the return of my primary machine, I did a fresh OS X install from the DVD (using the option to copy over my home directory from the backup machine in target disk mode), then used Software Update to install iTunes 7.2. The problem is that TV shows purchased from the Apple Store will no longer play on the primary machine, though they will on the backup. Double-clicking on their titles simply does nothing. In addition, TV shows cannot be added to the library after having been removed, and trying to open them in QuickTime Player causes that program to quit silently.

Music, including purchased music, plays fine. The file paths shown in iTunes for the video files are correct, and permissions look right as far as I can tell. iTunes claims that it has been authorized successfully, though I have tried deauthorizing and reauthorizing it a few times just to be safe. I can't think of what else I could be missing. Any ideas?