black screen video

Lately, when attempting to play some videos purchased from iTunes (in Protected MPEG 4 format), neither iTunes nor Quicktime Player will play the videos. Double-clicking on the file in iTunes (or dragging the video from the Finder onto the QT icon) results in a black video screen, but no video or sound. Moving the scrubber does nothing. I tried restoring one of the files from a Time Machine backup volume, and repairing permissions: no dice. I also tried to log out of the iTunes account and log back in, but that didn't help, either. Any ideas?

Alex