Without exact system info and other technical details like your QT settings from the system control panel, whether you use specific tools that simply may block access and so on nobody can say anything.
I do see some type of issue with Quicktime. Not sure what it could be.
Did a clean install and loaded it from Apple site.
Going to re-install.
I opened up all the other programs I have running and they all load Quicktime fine.
For some reason After Effect is not seeing the H264.
Not sure what to look for.
Can you render a QuickTime using other codecs besides h.264?