You can only read the RSS feed of your own community. If the provided team ID (tid) is correct, for the feed to work you either have to be logged in or add the servicekey of your community to the URL, e.g. http://classic.comunio.de/team_news_rss.phtml?tid=12345&servicekey=1CA3D5C1-861B-44AB-907F-9E086FAD2C18