Hi Antonio,

Firstly, May I know how author/you create the blog post in your site? Seems like there is a buch of blog post doesn't have author assigned http://take.ms/Ql7YC
Perhaps, do you recently delete some user in your site? Can you re-assign the blog post that do not have the owner and set the 'Owner for orphan items' http://take.ms/IDfp5