Without knowing anything about your source files nobody can tell you much. Start by flushing the AE caches.
It's an .mp4 filetype.I have been working with this file for almost 3 months.There wasn't any problem.But within a week its starting showing unsupported format.I can't even replace the file.But no problem on importing .mov files.
Thank you for your reply..
Did ypu try Mylenium's suggestion? Under preferences-> media & disk cache you should clean both options and see. if that does not work try reinstalling Quicktime
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