From the latest release notes of Plex Media Server 1.30.1.6497 which is availabe in the Beta channel:
(macOS) Some endpoints could return errors on macOS versions older than 10.15 (#13946)
This apparently is the fix for the “Server Settings are Unavailable” error. I’ve put the beta on one of my servers and so far it has fixed the problem.
Here’s the whole release note for this release:
You can either try the beta or wait for it to be released to the Public channel.
