you are a user of After Effects. There is a serious bug in the current version of Quicktime 7.4 that causes renders in AE to stop pre-maturely and forces AE to issue a permissions error.
Aparently this is tied to the DRM changes that went live in the current 7.4 version of Quicktime.
I have done a bunch of output from FCP 6.x using 7.4 and I have had no issues whatsoever. The issue has only been reported in After Effects so far.
UPDATE: Seems that it is causing an issue with streaming quicktime through the browser plugin - I have had it fail on Safari, Firefox and IE7 so far. - Bottom line: DON'T UPDATE TO 7.4. If you already have, consider using Time Machine in Leopard or a restore point in Win XP or Vista to roll back. Sorry, Tiger and previous OS X users - No fix for you :(