Hello, sorry for double posting, but I found out what was the bug and it's not caused by the game, actually it was an error caused by a software bug in my router that was recently fixed by my ISP.
But as I said, when I play using PPTP to my server and connecting to the game via the local ip (thanks to the 'vpn') I have no issues, which is really weird.
I got a bug that only occurs since I installed 0.15.0 (still here in 0.15.2). I created a map using the "Rich" scenario from my computer, then I uploaded it to my server (an Ubuntu 16.04 x64).
I'm playing with 3 of my friends, all of them can play without any problems. The thing is ...