First, if you submitted your podcast in the past, updating the language should fix the problem in a few days. If you've never submitted your feed to iTunes before, then read the next paragraph.
Make sure the URL you enter has no spaces in the front or back, and includes http:// in the front of the URL.
Update your Desktop iTunes to latest version. They reportedly fixed that problem, as it was a bug in the iTunes desktop software. If you still have the problem submitting the feed, close iTunes completely, reboot your computer. Then re-load iTunes and make sure you are signed into your iTunes account and your CC on file is up to date. Then navigate to the podcasting section, click the link "add podcast". (DO NOT GET TO THE ADDPODCAST PAGE BY CLICKING A CONVENIENCE LINK LIKE THE ONE IN POWERPRESS AS THAT DIRECT LINK IS PART OF THE BUG).
Older versions of the iTunes desktop software would not let you navigate to the add podcast unless you had an iTunes account signed-in and your CC on file was up to date. The direct link to add a podcast from PowerPress or other web sites/apps will fail as the bug in newer versions of the iTunes desktop software bypasses the authentication steps thus causing time out/404 submission errors randomly.