Piggybacking here--I'm having the same issue, but I'm not using Dropbox to host anything.
I updated my feed with the new-feed-url tag a couple of weeks ago. My host had already switched off the old domain (without telling me) so I had to ask them to turn it back on again for a couple of weeks. It appears they did, but now it's back off, and the iTunes Store never grabbed the new one.
My current, functional feed is http://philistineradio.x10.mx/episodes/Philistine%20Radio.xml.
What can I do?
It's better to start a new thread when you have a question; it can get confusing when people tack onto an existing thread as the answers can get interleaved.
iTunes is looking for your feed at http://s3.x10premium.com/~philisti/episodes/Philistine%20Radio.xml and of course not finding it. As you are unable to reinstate this feed all you can do is to email Support at podcasts 'at' apple.com and ask them to transfer the Store to the new feed.