Make sure to update your software to the latest version. AE should be version 11.0.2
What format and codec is the footage you're trying to import. What happens when you import something else (like a JPG)?
Actually, it would work best if you could answer all of these questions.
I found the problem. For some reason my camera records in 24.5 fps (very weird?), so after I changed that it worked. Except one thing, .mov files doesn't work. And let me answer those questions for you:
I use After Effects CS6 188.8.131.52, I'm using Mac OSX Mountain Lion 10.8.2, I'm trying to import .mov files, but it wouldn't work (that's my problem), so I changed to .mp4, but I would rather use .mov. Whenever I play the footage, whenever it shows up on the preview screen, my AE freezes. I do not use any thirdparty effects or codecs. My Mac-specs are:
3,4 GHz Intel Core i7 processor, 16 GB memory and an AMD Radeon HD 6970M 2048 MB Graphicscard.
Rest is just no, except the last one. I make a new comp and import files. Now, if I preview the imported file, my AE freezes, but only if it's a .mov file. And if I drag it into my timeline, it can stay there, but when I try to RAM preview or play it, it freezes.
Hope this can help you find a solution.
I have a feeling it's the footage itself. It could be some odd media container and codec, but at the moment, we can't tell..
What kind of camera did this footage come from, what is its media container (Quickitme, MXF, MP4, etc.), what is its codec (ProRes, PNG, H.264, etc.), how did you get this footage into your machine, what did you use to dtermine the odd frame rate, what did you use to change the frame rate and what kind of video editing application are you using?
Without this kind of information, we're flying blind.
Now, if I preview the imported file, my AE freezes, but only if it's a .mov file.
What codec do these rogue .mov files use?
To be honest, I don't really know. But I found out the framerate was 25, not 24.5 FPS. My camera went out of power when uploading, so the framerate must have bugged out, but I don't know, it's good again (I see the framerate after I import the file). It's a SONY AVCHD Handycam, 10.2 Mega Pixels, HDD, HDR-SR12, CMOS, and I'm using a Handycam Station to import it to my Mac. I'm not so good at cameras, so I just listed all the things it says on it's side :-) It uploads in Quicktime (.mov), and I think it's codec is H.264, but that's what my friend says (can I check it somehow?). I'm not using any videoediting applications other than AE for now, but I do have the Production Collection (the video editor).
Once again, hope this can help.
If you're having problems with QuickTime and the hang on initializing MediaCore, see this:
I suspect that you're hitting the issue with blocked TCP communication.
And another thing, it probably has something to do with the QT32 server. The only way for me to re-open AE after I forcequit it, is by stopping the QT32 server in the Activity Monitor. But I tried turning my firewall completely off, and it still didn't work.