I am still getting an error 404 for my feed

The Recent Global Posts Feed is still showing an error 404 through a Feed Validator and therefore is not being updated through services such as RSS Graffiti and Feedburner.

I have re-installed the plugin as per the directions on the plugin site, reset permalinks from fancy to default and back to fancy, and have verified that the permissions are correct. Someone had mentioned iframes being the problem, but I had those removed and still had the same issue. Additionally, the individual site feeds show as valid even with iframes embedded in them.

After all this, I am still getting the 404 error. Does anyone have any fresh suggestions?