Is a very irritating error and seems to be closely related to the NAT Negotiation error that frequently popped up with Company of Heroes.
Things that we changed that seemed to fix it (although temporary)
Check file integrity (Steam file checker)
Disable Windows Firewall
Make sure an exception is in place in Anti-Virus
Port Forward 23232 UDP/TCP
Not sure if it was an amalgamation of a few of these that were allowing us to proceed or that if it was just dumb luck that the error wouldn't show up. The issue is definitely intermittent and seems to occur between the host of the lobby and an incoming player.
The bug is only being seen during a lobby and causes both the host and incoming player to crash out to the server list. Attempting to re-enter the lobby brings up further connection issues but is of little concern, can easily remake.
Does anyone have any further possible solutions to this issue?