Forum Fatal Error on Megaupload.com Links
 

This is THE strangest thing I have EVER seen. My vbulletin forum is discriminating against MU links! Everytime I (or other users) post an MU link I get this fatal error:

You don't have permission to access /forum/newreply.php on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

Any other link works fine... what to do??