I posted this to another thread. Let me know if it helps :
SO.
I just figured out mine.
PROBLEM : The original media had a glitched frame.
SOLUTION : I re-cut around it and exported successfully.
My extended process for solving this is below:
I'd been trying to export and it didn't matter how I did it, through Master File or Vimeo or anything, always had a problem.
So I duplicated the sequence and tried deleting big chunks of the sequence and just exporting bits and pieces. The first half exported fine. Most of the end exported fine. The problem seemed to be somewhere in the middle.
I wondered if the software had exported anything successfully and so I went into the project file (right-click on project file, select "Show Package Contents") and found the export (Project File -> Event Name -> Shared Items) and I noticed that it ended at a particular place, right around the middle.
I found the place in the sequence and went through it frame-by-frame. It went black for a few frames. Weird!
So I checked the source media outside of FCPX and sure enough there was a brief glitch - just a frame or two - in the original source media.
When I eliminated that from the sequence, it exported without incident.
I hope that helps at all! Write back and let me know if that solved your problem.