I would look for that feed url in Administration > Advanced Tools > Url Manager
make sure it exists there
it is a "friendlyurl" that maps to a real url that should have a pageid and mid (moduleid) in the query string parameters
make sure those parameters are correct for the module id of the feed manager and the pageid for the page that has feed manager on it. for example if you moved the feed manager from one page to another it would have incorrect parameters and it would not work.
try the real url instead of the friendly url and see if you get the same result or not.