What changed on your system? New codec or codec pack? Software Update? Version of AE? What are the details of the clip at least from the info next to the thumbnail in AE's project panel? I don'thave the time to go to the site and download and check myself...
No changes to system, codec or anything else that I am aware of. AE 126.96.36.199. Clip info in AE: Millions of Colors | Foto - JPEG
Edit: I just borrowed a friend's graphics card and with it my normal graphics bugs have disappeared, but the problem with the file persists, so it was not the card's fault.
Next step will be trying to re-install the codec somehow.
Edit 2: Uninstalling and installing quicktime changed nothing. I don't know how I can uninstall and reinstall the specific Motion Jpeg Codec and I have no idea what else to do...
Brand new graphics card installed. No more glitches. Problem persists.
I'm inclined to wipe everything and set up a new system, but I'll have to find time for that.