Hi Paride,
I am really sorry for the delay of this reply.
Blog post preview will always display the preview post on the frontend page. I believe you are creating your blog post from administrator area and click on preview while composing. Hence when it redirect you to frontpage it will hit the following error,
"blog entry not found"
The reason why is because you need to logged in on your frontend with the same author that are associated with the blog post itself as only author of the blog will be able to see the preview. You can try to access the following url for the preview that I've created on your site without login and after login with the credentials provided from your previous reply above:
http://www.apiceweb.it/blog.html?view=entry&layout=preview&uid=65.369
Hope these help.