Oh, now that's a yucky problem. You can try to reset permalinks from Options/Permalinks, but it doesn't like that will do any good. Here's what we're going to do if replacing wp-blog-header.php and/or resetting the permalinks does not work:
1. Backup your WordPress installation.
2. If you have made any changes or hacks to the core WordPress files, place them in a plain text document, but do not apply them until you have confirmed that the feed is working or not.
3. Download WordPress again.
4. Delete every WordPress file on your server except those inside /wp-content/.
5. From the freshly downloaded copy of WordPress, upload every WordPress file to your server except for /wp-content/. Do not replace the copy of /wp-content/ that is currently on your server.
6. Run /wordpress/wp-admin/upgrade.php from your web browser.
7. With any luck, you feed should work properly now. If it still doesn't, then the problem is probably one of the plugins, or something deeper within your server.