rss feed problem

[محروم.كوم]

I am looking to allow our forum feed to public, I have enabled all the options in External Data Provider
but I get blank page when accessing : external.php?type=RSS2

on feedburner it returns : An error occurred connecting to the URL: Error getting URL: 500 - Internal Server Error

Most of the forums are open to guest viewing, so I think this should work the feeds, but its not. Why is it happening ?? How can I get this active ?
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