I've done some testing on our end here to see if I can replicate that, and currently it doesn't look like we're blocking any generic User Agents or header structures.
In order to test from here, I used the non-specific user agent, but with the same structure you mentioned, "nomeutente.altervista.org/indirizzo IP (compatible; MSIE 7.0)" and ran the following command from terminal:
"curl -A "nomeutente.altervista.org/indirizzo IP (compatible; MSIE 7.0)" https://stats.wordpress.com/csv.php\?api_key\=c4034998f7e2\&blog_id\=57986200\ &table\=referrers\&days\=30\&limit\=-1\&summarize"
This worked as expected and provided a list of referrers that would generally show in the stats module.
Moving forward there are a few things I'd like to check out.
Since I don't have specifics about the server, I'd like to test again with the exact user agent they're using for you, so we can verify there isn't anything specific with your site/server/IP address that's being blocked. Can you request that info from your hosting provider so we can test it more thoroughly with the username/IP they ran it against?
Also, can you have them try the same tests with different User Agents and let us know if those fail as well? This seems to be an isolated case, so we currently don't have much data to go on, but if they can provide us with more specifics about the requests they are making and the results we'll be happy to take a deeper look.
We'll be awaiting your reply.
Jetpack Happiness Engineer