Contact your web host!
A lot could have changed. Here are 3 examples, of hundreds of reasons why this could be happening...
1. Server administrator setup a firewall and is blocking outbound requests
2. Local server DNS is not resolving the host name where you're hosting you rmedia to localhost (127.0.0.1)
3. Someone installed mod_security and configured it to block many of the HTTP/1.1 protocol methods that iTunes requires.
You have three options:
1. Contact your web host to fix the problem.
2. Leave your web host
3. Host your media on a different infrastructure specifically designed for podcasting (like Blubrry Media Hosting).