Specifically, - Download the ISRG Root X1 certificate file from http://x1.i.lencr.org/ - Open the Keychain Access app and drag that file into the System folder of that app. - Find the ISRG Root X1 certificate in System and double click on it, open the Trust menu and change "Use System Defaults" to "Always Trust", then close that and enter your password to confirm the change (if prompted).
I don't have a mac, so I haven't tested this, but it should work. I'm surprised you are able to access this website at all, considering the issue.
I am also getting error 600 on Mac, using version 97.0. I have followed the instructions with the certificate, but the error persists. This only happens in version 97, all other versions reach the server with no issue.