The problem is that your site serves an HTTP 301 redirect to
http://9marks.org/feed/ when that URL is fetched with the FeedBurner user-agent (or apparently, whenever the user-agent contains "feed"). If you serve the same content regardless of user-agent (or, more commonly, you serve a redirect to the FeedBurner URL whenever the user-agent is
not FeedBurner) then FeedBurner will be able to fetch your feed's content.
- Maks.