I'm trying to set up a lan game on two computers, both on the same LAN. I own one copy of the game from GoG, which is installed on both. We can each see each other in the lobby, but when attempting to connect, we get a connection refused message.

This happens regardless of whether we set a password or not.

After trying a bunch of troubleshooting, the only reason I can think of that it might not be working is that whenever we try and create a game, the username given to both of us is automatically Solusphere, with no option to change it. This might be causing the issue?

I distinctly remember that local multiplayer with a single copy of the game was possible with the original BZ2. I bought this on GoG, which has significantly less modding support, for the sole reason that it doesn't include (or isn't supposed to include) DRM restrictions in it's games. So if it turns out that it's not possible to run a lan game with a single copy, I am absolutely going to be asking for a refund.

If it isn't that, can anyone tell me the ports I would need to set up port forwarding, or any other possible fixes for this issue?

I had the exact same issue. I bought another copy, since it is on 75% sale. That is the issue. I also thought this whole DRM free gig was supposed to let me play on the LAN with one copy, but even connecting through the GOG Launcher will not allow it. Spring for the $5 this time, this game is fun multiplayer.

GOG Team: if a game cannot be played multiplayer with one copy, could that please be noted on the store page for it? I pretty much only buy things to play with my son, so multiplayer and coop options are key to my purchases. Thanks for considering my request.

Can you confirm you actually got an old game to network in windows 10 by buying a second copy?

I just assumed game networking is dead for older games, and Widows 10 was the reason.

The old games i like allot, i have an old copy on disk and bought it again on GOG simply to not have to use the disk - because that is a convenience well worth paying for!

Could i then upload the old game disk to Steam and run one game in steam and the other in GOG and then it will network??

This remaster and the original game use very, very different netcode, but both still work in Windows 10.