It seems patch 2.0.4 from a few days ago has been causing a lot of grief for console co-op players.
Since Tuesday’s patch, many players have been reporting being completely unable to play. As soon as the second local player joins the game, it crashes. As you’d expect, players tried everything to fix the issue, including re-installing the game, but nothing worked.
There are many, many more threads like these over the past few days, so it seems the problem is affecting pretty much all local co-op players.
This only adds to the already existing issues for local co-op, some of which have been in place for almost 2 seasons now! The Infernal Hordes bug has been around since the start of Season 5. Meanwhile, the expansion raid, Dark Citadel, is not actually bugged, but simply does not support the mode. Players need to split up for the raid, which is not possible in co-op the mode since both players need to be on the same screen.