After Effect has major problems if you upgrade QT. To be safe, stay with 7.31 (maybe 7.4) I have had problems rendering and recently, this week even booting AE. My QT was upgraded by someone in our shop trying to be kind. If you have upgraded and are experiencing problems do this:
1. download QT 7.31 package
2. download Pacifist software.
3. Use Pacifist to install (basically overwrite) QT 7.5 or whatever you have upgraded to.
4. Contact Apple and bitch them out.