Hello guys, thanks for your responses. I did not have the chance to test it yet even if I am now at home, I'll do it later today or in the next days.

Anyway, for those who still have problems in updating Skype to 8.51, a friend of mine sent me an installer which I don't know where he found, but I created a link on Firefox send in case someone needs it. Just a pair of notes before the link:

1. you aren't forced to use Firefox to download this file, but you have to use a sufficiently updated browser. For example, according to my tests, Internet Explorer will not work with this server, even version 11;
2. I configured both combo boxes at the highest value available, meaning 100 downloads or 7 days before the link expires.

Download link: https://send.firefox.com/download/e482c8237e22cd6b/#FG9LN1Whuc4GZQTqs0GRCA

Bye!

Barbuz