I created a komposition and export it at Quickime (mov). I had Quicktime 7.5.5 The film plays good. Now I updated Quicktime to 7.6 and I get an error when opening the movie. Back to 7.5.5 everything works great.
Besides the fact that MOV is a container carrying codes - which you did not explain what codec you have used - it's generally a huge problem at Apple.
Updating to the newest generation of Quicktime player is somethig to consider really well, especially if you're in the middle of a project.
There are so many undocumented pitfalls you can easily avoid by NOT UPDATING unless something is really bad with your actual version.
Most people remember the flaws with version 7.4.x which made Ae useless for rendering sequences.
The hassles with Gamma- and Lumashifts is another story which Apple still does not have under their control.
If you need support on that, you're pretty alone in the dark.
Apples "mystery-mongering" way of running their business is simply contraproductive in terms of the Pro Application Users.
Please specify the exact message when you get the error. In particular the QT exit code (the number in the parentheses) is relevant here. Thomas is right of course, it's Apple's silly mess. You could however try to open the file in QT player 7.6 and save it from there, then try to open it in AE. Could just be some weird thing with stream ordering and tagging that gets sorted when re-saving it.