Can't play skirmish with friend, stuck at joining game

I'm just copying my previous post, because noone answered.

Hello there!

In medias res.

We can play private games (walker assault, hero hunt, everything), it works even when 5 of us are together. There's no issue with standard multiplayer or offline skirmish either. The problem comes when we want to play skirmish together - when there are only two of us and we want to play against each other, with AI teammates. We can't... actually just I can't. It runs well for everyone else. They can play skirmish, but when I try to play with one of them, neither can they connect to me, nor can I to them. It gets stuck at joining game / waiting for partner to join.

We have tried to solve it with basic problem solving things, they did nothing, then moved on to port forwarding, taht didn't help either. I'm connected to our router via an ethernet cable, I have the most recent drivers installed, I have tried turning it off and on again.

I'd like to get a solution for this, because we'd love to play skirmish together.