Unable to join game session

[hon3y]

Hello, im here to talk about the well known "Unable to join game session issue"
I wanted to add some informations :
-it happens only to me when they try to join me, but i can join them
-it happens even on another account
-it happens when we are on the same network (im playing next to them)

Dont respond me about port forwarding plz cuz i'll go crazy.