Impossible to start multiplayer game

Greetings everyone.

We tried to play Starcraft Remastered with a friend in 1v1 via

As soon as he joins the game, it is impossible to start it, the “OK” button is grayed out. When I am alone I can start the game. And we tried the other way around, he can’t start a game even if he’s alone in it, as a host. Of course I already played multiplayer multiple times so the issues comes from his side.

Of course the firewalls are ok etc…

I haven’t found this problem anywhere on the forums. Anyone has an idea to solve this ?
Thank you