Plutonium

Having this same issue, specifically with T5 Zombies. I was playing with one friend just fine, but another friend tried to join and received this error. When he tried hosting and I tried to join as the 3rd player, I was kicked out of the lobby to the title screen but the other two saw me as "in" their lobby for a couple of seconds.
Seems as though 2 player zombies via private match works but whenever we try to add the 3rd player it just gives this error. Joining an active server worked, but that's less preferable.