UE4 Flex: multiplayer is broken?

Does Flex not work with multiplayer? I am using Flex 4.16 and none of the demo levels work in the sample flex project on multiplayer. I’ve tested listen and dedicated servers. Since Killing Floor 2 used Flex, there must be a way to get it to work. Any ideas?

To be clear, I mean that the game will instantly freeze and become unresponsive when run with 2 players.