I managed to fix this issue by doing the following for a non-Steam copy.
- Launch the game while completely disconnected from a network, I unplugged my cable.
- Click the multiplayer lobby you want to use, I've only used LAN I don't know if Internet would work.
- When it asks for a nickname and you've chosen and it gives you an error, re-connect your network and try again, it should work after that and let you into the lobby. You'll need to go through the process every time but it means you get to play multiplayer.
Enjoy.