First in your description, it seems that you are using a slave site as a "Sub-Directory" and not as a "Sub-Domain".
You mentioned that you use the shareMasterSubdomains and the rule that you should use is "shareMasterSubdir" because your URL requested is
www.xxxx.it/automazione
One of the potential reason to have "403 forbidden" could be due to a problem of permission.
To verify the permission of you server, you can use the "hello.php" (see video 7) to verify if you can run php accross the symbolic link.
With FTP, verify that the /htdocs/public/www/automazione directory exists and that inside, you have the symbolic links to the "administator".
Put a hello.php in /htdocs/public/www/automazione
and put a hello.php in /htdocs/public/www/administrator
On internet, enter
www.xxxx.it/administrator/hello.php
You should get a page displayed.
Repeat with
www.xxxx.it/automazione/hello.php
Again, you should have a page displayed
Repeat with
www.xxxx.it/automazione/administrator/hello.php
If you receive a "403 forbidden", contact your hosting provider and ask him to "Follow Symbolic Link"