RSS-feeds: some don't work - why?

The RSS plugin works for several RSS feeds, but not for all. I tried these ones from www.heise.de, but they stay empty:

http://www.heise.de/newsticker/heise-atom.xml

http://www.heise.de/newsticker/heise.rdf

(The two on top of this page: http://www.heise.de/news-extern/news.html)

Why are they not working? I think, Atom-feeds can be treated by the plugin?

I'll see if we can replicate the issue here and find a solution.