The problem has continued to happen to me and others the past 3 days.
Note, the problem is NOT a white board, but rather that when a player attempts to join a 4 person game that was created before the player logged in, an error happens that logs the player out, resulting in a largely empty screen. There is no board, because the player never joined and the host had no opportunity to accept the player.
I am under the impression that Pat30 sent in a video of this happening several days ago.
Note that there is a workaround. The player can log back in and ask the host to delete and recreate the 4 person game while all the players are logged in. Then everybody can join.