Running into this same issue, even when using Hamachi. I can see all of my friend's games (either when using direct TCP/IP or if we're on Hamachi), but he can't see when I host.If we try to start the game when he hosts, we get the same error message.
I had similar problems once.
Did you try running both the games as Admin?
Are both your game versions the same? If you use a mod, are your mods the same?
Hard to guess what could be the problem. I can verify that despite the game's age and problems, LAN play does "work" potentially, I just did a few rounds last night. Possibly something to do with different game versions? The mod is definitely installed correctly on your end? It's always easier to troubleshoot these kinds of problems if you have 3 people, see who's game is acting up.
Hey! Cool to see you back around here