I have been trying to use a lot of random opponents.  I have noticed in the last couple of games that with two specified aliens chosen and the rest set to random, the game will not start and reports that "no opponents have been chosen".   I have to manually put in 2-3 new opponents from the list of potential opponents before I can start the game.  I have tried with all opponents chosen and no specific civilizations chosen this message also is generating.  It appears, though this from a small sample, that one needs to add a couple of specific alien opponents to get the game to start.  This is strange,  I should be able to choose all random opponents.