Thank you very much esmi! It worked.
Maybe for others interesting: Opening it in Google Chrome (never did it before) showed me exactly on which line the problem is. In fact, the theme caused problems. 3 entrys have been a video entry and in this category seems to be a problem (whatever the reason is).
Now it’s working. But one more question: The feed is showing me: “This XML file does not appear to have any style information associated with it.”
How can I add some style information?
