Hey pewp3wonline, as well as forwarding the ports and what you have already tried, there is a possibility of either you or your friend having double nat, but if this was the case it would affect most games you and your friend were trying to play together and as you have said this only seems to affect Anno 1800. There is also the possibility the save file may be corrupted.
Do you both play with mods and do you have many of the DLCs activated on your save?
My best advice would be to get in contact with us and make a support ticket. We will ask you what troubleshooting you have already done so please be specific, then we will ask you for a screen shot of your port forwarding settings. After that we can escalate this to our technical team to look at and advise. If no solution is then found, we know its either a problem with the game or a corrupted save file, so if you can I would also attach the save file to the case too.
You can make a new case her...
Read more