I can imagine some setups where this is possible, depending on the naming resoultion of your server. For example, it might be possible that the url http://www.myserver.com/myrssfeed.aspx works on your client computer, however it will not work if exceuted on the server directly.

If you have direct access or if you are able to connect remotly, I would check first whether you can open your portal (and your feeds)  with the same url in a browser on your server.