admin/store/uc_rss_feed creates an xml file and also prints the xml document to the browser screen.

If you cannot see the xml file check/update your browser.
If you get 'access denied' it is lack of permissions full stop.

Set correct permissions, Log off clear your cookies etc and log on again.
Try another browser if problem persists, i favour firefox and googlechrome.
If all that does not work check to see if you have hackers, for example russians etc becoming members of your uk website, there is a major security risk at present where becoming a member and changing the password allows code injection, you can then mess up permissions and naurally the owner is locked out while you hack using uid 1.