[2023] Mods Break Multiplayer Connectivity - Even when turned off.

Do you end up with the same checksum in the MP lobby screen as your friends do?

And, can you test with the Cross Store MP option in the launcher's Game Settings menu?  

Have all players check that all required Steam ports are open (NOT forwarded, open) in all their firewalls:

Steam Support :: Required Ports for Steam

support.steampowered.com
Good luck!