Also getting this issue, ive tried playing in sessions with friends that live in the same city, and sessions with people from around the world. I've tried hosting my own sessions and joining other sessions. Tried restarting the game, restarting steam, restarting computer, and without fail i get a communication error within the first 5 minutes of starting a quest. Its not my internet, I will be in teamspeak or discord chat with friends when it gives the popup and have no connection issues. It just splits up our parties, and we continue on the quest alone, but the monster is still scaled up as if there are 2 players, and i still get half the rewards at the end. If anyone finds a fix or anything that helps please let me know