I don't really know if I am missing something. I have configured the server to accept L2TP connections, set a new PSK and created users with "user & password" authentication.
Also I looked for answers at the forum and I found that I had to open the ports 4500 and 500 (UDP) and so did I. Anyways I have disabled the firewall, just in case.

Then I have configured the android client as well but I get always an "unsuccessfull" message. I have also tried to connect my device to a server that I found in https://www.vpngate.net/en/, using L2TP and it worked perfectly :/.

Here you have attached a few images of my configuration (both computer and android):
Server L2TP config.

Server user config.

Android vpn connection settings.


Thank you very much in advance =D !