For some reason I don't understand, a real error message is in the log file. From my experience it almost always says that the game(s) you tried to join are full. Since that's obviously not the case in the UI, it either means that a) the error message is wrong, b ) the master server is *way* behind on updating game player counts, or c) the UI is showing the wrong number (for example players that are currently connecting might be counted against the player limit, but not shown in the UI as in the game)
