As simple as it sounds. Both of us have the Xbox One. Both of us are on the same network (his is wired in with the ethernet cable and mine is wireless). Until last Saturday, we could join each other's fireteam and play all we wanted. Since then, whenever we try to join it says "Unable to join target player. Please try again." It does not matter who sends the invite or where we are in the game (in the Tower or in Orbit doesn't work either). Has anyone found a fix to this problem (preferrably on Xbox One)?