From the Cowbots and Aliens folks -
"By way of an update - We have discovered that the bug you are seeing occurs when more than 4 players try to join the game from the same IP address. It is a strange bug that was introduced recently by our multiplayer backend partner Photon.
We have reached out to them and will be working with them to address the issue. For now it would be best to limit the number of people playing Cowbots in one of your locations to a maximum of 4."
That is a really strange bug! Glad they've tracked it down though, will let you know when they update!