I had a similar problem when trying DS2 on a LAN - the only workaround I can find is the following: (although it seemed a lot of hassle so I've not actually tried it!!)
1. Connect to Hamachi.
2. Disable your primary network connection through Network Connections.
3. Launch DS2 and start hosting a game in the LAN lobby.
4. Tab out and re-enable your primary network connection.
5. Reconnect to Hamachi (power button twice).
6. Have other players join your game.
7. Play!
Note: Incase you encounter the "this name is already in use" when trying to join the LAN lobby, everyone involved rebooting their computers will solve the problem.
If this doesnt work - go out and get Titan Quest and Titan Quest: Immortal Throne - its LOADS better!!!!!
Hope this helps....
StevieP