It's been awhile so don't hold me to it but I think this is right. Go to your Library, select Preferences, and then move the following file onto your desktop:
After that start iMovie back up again (make sure you closed before you started this) and it should rebuild and your problem is solved. If so you can delete the file on your desktop. If I have gotten this horribly wrong just drag the file back in there.
For what it's worth I now use Onyx to clean up my computer, including permissions, and have had zero problem since.
Hope this works and let me know.