Solution:
In my case it was the proxy blocking the communication with WordPress server.
I fixed it by following this guide.
Basically I had to define these rules into wp-config.php
:
/* Configure proxy Server */
define('WP_PROXY_HOST', 'x.x.x.x');
define('WP_PROXY_PORT', '8080');
define('WP_PROXY_USERNAME', '');
define('WP_PROXY_PASSWORD', '');
define('WP_PROXY_BYPASS_HOSTS', 'localhost');