Bo3 custom zombies, unable to play with friend

Hello , any opinion would be appreciated . Me and my friend both have t7patch , when we try to join eachother an error gets and says “game lobby not available” my nat type is open his is moderate , what we have to do to play some custom maps together ?

"As of recently, the game will crash in online/private matches; but checking the repo, the game has not been updated for a while, so this seems to be a steam client issue.

If you are experiencing issues with playing in private zombie matches, when the game crashes upon starting; switch your steam version to the beta client.

Switching my game to the Steam beta client fixed the issue immediately."