timex550: Ok, right to the case.
So, me and 2 friends have been playing HoMM 3 with HD Mod over the LAN. I was hosting a game and everything worked just fine for a long time. But a few days ago we encountered unusual problem. When I create "room" (so basically - host a game) others can see my game and join it but I CAN'T SEE THEM or start the game. I also can't see any messages from connected players or even 'system' that someone have joined.
So we tried with other person hosting the game. I can join the room but can't connect when the host loads a game. It shows "Error sending data".
Do you know what the problem might be?

needlestorm: First guess would be a firewall issue. I'd check that you are allowing both incoming and outgoing traffic on the required ports.

timex550: Yeah, we tried turning firewalls off but it didn't help. Still, before, when everything worked, our firewalls were on (of course with added exceptions).
And another thing - we tried launching HoMM without HD Mod. What happened was - when any player tried joining my room the game showed "Error connecting to that session. Try again?". Any ideas now? I'd be glad for any help.
Just to be clear - we are all in the same local network and our computers can see and access each other. And I didn't change any system or game setup between the last time multi worked and first time it didn't.