I see... if you deactivate the "Split by: Year" option on Settings > Reading (click the link Settings after the "Include post types: Posts") then your sitemap index will show this one http://www.nuvoluzione.com/sitemap-posttype-post.xml which works without being redirected. I think it's the year in that URL which causes an Nginx rule (or are you using a redirect plugin?) to redirect to the home page.
For example, if I try to open up a normal year or month archive page like http://www.nuvoluzione.com/2016/06/ I would expect to see a list of posts published in that month but instead I'm redirected to the home page too...
You'll need to find out what causes this redirect, or switch to "Split by: none" as suggested above. But be aware that this makes the http://www.nuvoluzione.com/sitemap-posttype-post.xml a slow one (Google spider does not appreciate that very much) and at some point in the future, if you've published many more posts, you'll run into a PHP memory problem, causing a blank sitemap...