Unable to play the Endgame in Couch Co-Op

My girlfriend and I reached the endgame yesterday. However, when we tried to start a map, the Atlas did not appear when we clicked on the map device. Instead, we only saw the Act maps. When playing alone, there was no issue opening the Atlas or running maps. As a workaround, I started a map alone, exited the game, and let her join it. This allowed us to teleport to the map together, but after a while, the screen turned black, and we couldn’t do anything except teleport out of the map. Since she had died a few seconds earlier due to lag, the map failed instantly after I used the teleport.