Unable to join friend's private/public game. "Room is full."

[PrincessKaguya]

Me and my friend are trying to PvP one another, but when we attempt to invite the other player it says "The room is full." When it is clearly not full. We tried opening the game to public. Did not enter successfully. We set the game to private with a password. The other player entered all the information and still was unable to gain access to the room still claiming "The room is full." What's the problem here? We are both level 40 / 41