FBML Static Error: Failed to fetch required static file

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FBML Static Error
Failed to fetch required static file
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I've crawl internet for solution and couldn't find it. I guessed the problem must have something to do with Callback URL. As of now, I set the URL like this

http://www.myDomain.com/folder/

Prior to this, the URL is set to

http://myDomain.com/folder <-- Cannot set the URL like this. Facebook doesn't like it

Related Links:
http://forum.developers.facebook.com/viewtopic.php?pid=66544
http://forum.developers.facebook.com/viewtopic.php?pid=62558