I can confirm your observations. The iTunes application cannot access the feed, although a browser can. I have seen other instances where a server apparently would not respond to iTunes where it would to a broswer, but no satisfactory explanation has emerged. One poster found that it was a permisions issue on the folder on the server where the feed was placed, and that moving the feed to another folder solved it (though of course the Store would lose contact with the feed if you do this) but I don't know whether this is related to your and other similar issues I've seen.
I can only suggest that you contact the people running the server: server logs may show what happens when a request originates with iTunes.