Zombies (only) crashes on accepting invite

[necessity420]

Hey! Whenever I invite a friend or a friend invites me in T6 Zombies, the game crashes for whoever accepts the invite. It just closes without any kind of error popup or message. There's no crash dump in the AppData\Local folder. This does not happen with multiplayer. I've forwarded port 3074 UDP on my router, and also tried the connect <ip> method, but it fails to proceed after the connecting stage on my friend's PC. Any ideas?

[Resxt]

Are you really sure that you're both in ZM?
Last time I heard this the other friend was in MP while trying to join a friend in ZM

[SwaFromSteam]

Same for me.