When attempting to join a mission while you have an open matchmaking lobby in Marvel’s Avengers, other players will attempt to connect to you. If they successfully connect to your game, your team will go through the mission with you to its completion. Before that happens you have to connect with them. An unfortunately common sight is having a failed to join session error pop up on your screen, and you go back to the main menu of the game.
The best way to ensure you don’t encounter this error is to double-check your internet connection. If you’re having problems, they could be happening there, but it’s unlikely. The next step is to give Marvel’s Avengers a fresh restart by turning it off and then on again before attempting to try another match. Should you continue to encounter this error, you can try to create a game without matchmaking with other players.
To prevent matchmaking from automatically happening you need to alter the settings in your options. Go to the main settings, and then go to the gameplay tab. The third option should say matchmaking, and you need to disable it. Now, whenever attempting to do a mission that allows multiple players to do it together, you won’t automatically do it with random players. Return to the war table after disabling this setting, and try to create a match again, and the error should no longer show up.