Well, I had the same problem and solved it. Here’s the story:
I did a screen capture with QuickTime (most recent version as of 9/19/12) and was trying to import the .mov files that it created. The raw ones wouldn’t import. Copies wouldn’t import. Copies on another HDD with a duplicated project saved to that other HDD didn’t work. I even tried exporting from QuickTime as a .m4v – same error! I was about to give up.
However, by using another import menu (sorry – not sure which one) I learned that the issue was an unsupported frame rate! The video QuickTime had created had a frame rate of 15.002. I ran the file through Compressor to change it to a 15fps frame rate, and it imported normally.
So, if you can’t figure out why you’re getting this error, check the source video frame rate.