My girlfriend and I are also experiencing the same issue. We try to invite each other into a game, she will get my invite, but I never receive hers. When she receives my invite, she isn't loaded into the game and all I see is the "waiting for players" spinning loader animation.
I've run through firewall settings and can't see anything obviously incorrectly set. Does anyone have any ideas?
I made sure the firewall allows Anno 2070, did all the port forwarding, and ended up using a VPN and it worked. Then it didn't let me save my files, so I manually had to set save files to non-read only. Now I can save, but now the game doesn't recognize the save files. In the whole process, we started a new game 3 times and wasted our whole weekend starting over and over again.
I have exactly the same issue. I tried FW/AV shutdown but nothing works. We used to play Anno 1404 (GoG version) and that works well. But I spent several hours trying to connect 2 PCs that are in one network. Accounts are visible in contact list but it looks like only one can be fully online - can send messages send invites but the other PC sees it but cannot reply nor accept invitation. I managed to somehow connect once but I was unable to repeat it so far.