Jeroen recently notified me, that my blog's feed at http://lekv.de/feed was broken and returned 403 - Forbidden. Looking into the directory Blogofile renders the site into, I found, that the feed is contained in a file named index.xml. However the VirtualHost configuration of my blog only contained index.html in its DirectoryIndex directive. Adding index.xml to the list solved the issue.

Thank you Jeroen for the hint!