on http://www.sensorgrafie.de/blog/tag/feuerwerk/ The extra div is after the posts so I am not sure how it gets there.
Check the content editor of the post page and also the post content HTML Insertion areas
The blog page appears to use two columns, but there is only one post. Can you try to switch it back to one column and see if it makes a difference.
Also create a test page and redo the post page in there to see what happens with that new test page (give us a link)
Ignore the <i> I don’t see it anymore.
if you have the problem on a different blog, please give us the link so we can check it and see it there is a similarity