I've been playing with my Friend (He's on Steam) over LAN Mode for a few days.
Now that another friend wanted to join we went to the Online mode.
But now i can't join him anymore. While other friends on steam can join his game.

i deactivated "upnp" in the options.ini file because the game would start as a Blackscreen when i first installed it.
Now that i've set it to true again i still get the non connecting issue.

Edit:
After a while i found out that you can only connec to players with the same Difficulty setting. because i set mine to Veteran i couldn't connect to any other. It would always say "Unable to connect" without any details why... GG

Just the way it is for the game I guess. It would help sometimes for a lot of stuff if more info was given than just "can't do this or that" with nothing else.

So i Managed to connect to my Friend's server on Steam.
But he cannot connect to my server. wtf?

Port Forwarding is set correctly.