Hi, Sumit. I have the same problem with LR 4.3 on a Mac (running 10.8.2 Build 12C60) -- Starting yesterday, LR has started to refuse to import .MOV files, from 1 second in length to 1 minute from my Nikon D4. The last time I imported a video file was Jan 30 (~30 days ago). Some of the videos are the same format that I've imported many times using the same exact procedure, and some are 720p 60fps, which I haven't used in a while. None import, although the still images shot before and after them do without fail. The error I get is after a 1-2 minute delay while LR "thinks," and then says "There is a problem with the video file." I do recall installing Java updates pushed from Apple.
The videos play fine in Quicktime 7, Pro, and using a space bar in preview mode.
I've read your post above, cleared my video cache (command completed quickly, but the folder still has GB of files in it), checked permissions using disk utility, manually renamed the folder to force recreation, but no change in behavior.
I went to a gallery created a month ago into which I successfully imported the same type of file from the same camera, removed it from the catalog, and tried re-importing it. It failed also.
Thoughts?