Error message trying to do multiplayer

[Vixen525]

Been playing a lot of multiplayer with one of my besties and this time I'm getting a message.
"Join Game Failure"
"There was a problem joining the game"
"JoinPlatformSession: Failed: 6"
I tried restarting Steam, then restarting my computer without luck. We both play on steam, first time we tried multiplayer it didn't work until we both enabled crossplay, but now we've been playing together weeks and suddenly this.