Multiplayer Direct IP issues

[Project-404]

Hello

I have an awkward problem with a multiplayer game: Dungeon lords V1.4
It's a direct IP connection, and I am hosting, I turned my firewall off, but not the antivirus. My IP I found through the 'My public IP' webside. My friend then attempted to join my game, enterring my IP adres and clicking join game.

First it didn't work, and then (after me turning my firewall off and him adding dungeonlords.exe to his exception list) we suddenly got connected.
The bad thing is we then had to turn the game off due to circumstances, and about 10 minutes later we tried again and it said it couldn't connect!
We tried to mimic the exact same configurations we had when the connection worked, but still nothing...
We are permanenetly keeping communications through Skype and he said that his game showed an in-game message, something like 'Negotiating with NAT' and after failing 'Check your NAT fort - or something like that'.
Both my fort and my friend's is 2606, this was automatically filled in as soon as the multiplayer screen opened.

If any additional information is needed, post and I will add it.

Many thanks in advance, this is a truly great forum,
Project-404