I installed ATM3 v6.0.0 via the twitch launcher and opened a server via a website for me and a few friends to play on, we were unable to join the server and I contacted the websites support, after a few attempts they were unable to assist me and say the issue is client side rather than server side and they cannot help me.

When I try to join the server, I get error messages, sometimes it will say "Fatally missing registry entries" and sometimes it will give me a long error message, I have included screenshots.

I have the same problem

There is a mismatch between the client and the server registry. I will close this for now