>UPDATE: QuickTime 7.4.1 Fixes Rendering Bug
Just like with QuickTime 7.2, the newly updated QuickTime 7.4 seems to be creating problems when rendering, not only from After Effects but even from FinalCut Pro. People report getting a errors like:
After Effects error: opening movie – you do not have permission to open this file (-54)
There are some suggestions that this is the movie rental DRM in iTunes that tries to verify the copy-protection in all open QuickTime movies every 10 minutes.
The work-around is to render to a file sequence, such as TIFF, and then either assemble that into a QuickTime movie in QuickTime Player Pro or by importing the sequence into AE and rendering out a movie from that sequence.