First try validating the feed. You can do that at a number of places...this happens to be the one I use;
http://feedvalidator.org/
If the feed validates then you have a unique problem as I have not read anything about people having this particular problem with the WP2.1 and WP-Autoblog. I did find a little something in the WP support forum but that seemed to be around atom type feeds. You can read it here:
http://wordpress.org/support/topic/104330