@Akigis,
The only other thing that can cause desync is having garbage files in <game folder>/data.
Go there and delete the Index.bin file and the RMGTemp folder.
Make sure both players have those files deleted and try again.
Regards,
Jordan
Please tell me, is it possible to change the settings for teams in the map editor? When I set up the teams, I get this warning:

How can I make it so that my friend and I are on the same team against an AI team?