Can't play zombies with a friend using mods (T6)

[Alwaroski]

In summary, I can play standard game with a friend but can't play when activating a mod. Usually, he just creates a custom match and invites me or I join him when seeing the green square icon in his name. I can't do that when the mods are activated. For context:

• We have exact same mod.
• We tryed different ones. No one worked.
• We can play solo all those mods normally
• We can play together without mods
• We both verified ownership of the game
• In my case, I managed playing a mod zombies game from the server list twice this week without any trouble.
  I even tryed using the command "connect <ip>" (we are on same LAN), but didn't worked, so I don't know what's really going on and why it affects only mod games. I hope someone knows what to do. Thanks in advance.

[bazinsky]

Also currently having this issue. We additionally tried running a private server but that also did not end up working for us. Would love any further info on this.