Simona Sulis

I think that in my case the problem is not the router, because Minecraft 1.12 works without problems, although Minecraft 1.12.2 doesn't work. Something must have changed in this new version.

Meep Games I've just checked the solution that you've suggested; unfortunately it doesn't work. The problem still remains.

I was having the same problem with a vanilla server version 1.12.2 (online-mode=true), my friend joined the server but after few seconds he was disconnected, this happened every time he tried to join. I rolled back to 1.12 and that way it works fine without disconnecting. Previously (before the version 1.12.2) we never experienced any problem.
PS: I also tried snapshot preview 17w43b but he had the same disconnection problem.