We are both on steam. I suppose i am the problem since my friend and a third guy can create and play the game without problems

I both reinstalled and verified the game. Still can't join a custom game without making the host crash the lobby

If you and your friends are on the same platform, I don't see why you wouldn't be able to play together. Adding the game to the firewall should this these issues most of the time. In this case, try to verify the integrity of the game's files and try again. Either way, I would recommend setting up a dedicated server through a network hosting app or finding a cheap VPS server as an alternative solution. Most of the time, when players have to host a lobby by themselves, these types of issues appear.