The only possible issue I can see in the feed is within your 'enclosure' tag which reads like this:
<enclosure url="http://cdn.ehrensache.fm/podcasts/es001einfuehrung.m4a" length="1307184" type="audio/mp4" bitlove:guid="http://ehrensache.fm/podlove/file/3"/>
The 'bitlove:guid' tag is non-standard and has evidently been put there by the method you are using to create the feed. Although the iTunes application is ignoring it and is happy with the 'enclosure' tag it's possible that the Store, which tends to be more touchy, is thrown by it. I don't know how you are going to be able to get rid of it if you are using a creation method which automatically puts it there, but I suspect you would be better off without it.
Otherwise the media file itself seems OK - it's not forcing a download, it checks OK for byte range requests, and plays OK when checked in a browser. We've sometimes seen issues where a server will not react properly to requests from the Store when it's OK with everything else, though I've not seen any explanation, so that might be a possibility - it's something you would have to take up with the people running your server.
