Hey Dennis,
I am sorry that delay of this reply,
I have do some comparison with following RSS feed URL :
http://www.haufe.de/xml/rss_129130.xml (one of your feed URL)
http://demo.stackideas.com/?format=feed&type=rss (our demo site RSS feed url)
Then I compare with this RSS feed URL, only this validator reported this following error :
http://www.immonewsfeed.de/rssValid.php?iKey=1dd35a979940a6ac06488421e34566a5_334&anzahl=25
This feed is valid, but interoperability with the widest range of feed readers could be improved by implementing the following recommendations.
Feeds should not be served with the "text/html" media type [help]
Your feed appears to be encoded as "ISO-8859-1", but your server is reporting "utf-8" [help]
line 641, column 0: Missing atom:link with rel="self" [help]
I believe because of this causing the feed importer can't import the feed through this RSS feed URL, you can see this 2 RSS feed URL is work without these 3 error above.
http://www.haufe.de/xml/rss_129130.xml (one of your feed URL)
http://demo.stackideas.com/?format=feed&type=rss (our demo site RSS feed url)
Actually this is out of our control regarding this RSS feed URL.