If you have an MP4 movie whose icon says ERROR in Usher, but you're sure should work, here's a possible simple fix: change the file extension to .M4V instead of .MP4.

(Background: QuickTime X handles these files just fine when named .MP4. QuickTime 7, though, does not—and Usher uses QuickTime 7, for a number of reasons. Once renamed, though, they should work just fine in QuickTime 7—and hence, in Usher.)