Found it. There was a repeater hidden at the bottom called "Posts from single author". This is the where condition in the repeater configuration:
BlogPostAuthor LIKE {% QueryString.GetValue("author") |(identity)GlobalAdministrator%}
Is it possible to insert some macro which tests if QueryString.GetValue("author") is a number, if not just to return to homepage ? Because there are two more repeaters for year and month...
http://prntscr.com/jz30sl