@Avalanche_knivspark: Ive been experiencing this issue again and again, and it seems especially hard for my son to join with a invite. We only play with invites so i cant say how it works when joining a random server and ive never seen a option for joining a friend without a invite.
But it really feels like the delay between clicking “accept” and switching back to the game affects this issue.
I usually have my left hand resting on the alt+tab-keys while i click accept on the invite and it usually works for me on the first or second attempt, but my son is not as fast and can sometimes require 10+ invites before he can join, unless i help him accept.
To be more specific, it is usually less than 2 seconds from clicking accept to entering the game for me, seemingly before the game loads that square dialog window “joining game”.
So it seems that when i switch back to the game late, the dialog window is already there with the message " unknown error".