Unable to Sign On

I’m unable to sign on to https://app.plex.tv.

“There was a problem signing in to Plex.”
“Invalid Credentials.”
“The password or username is incorrect.” (They are not.)

These pop up in an endless loop, in no particular pattern I can discern. Occasionally I appear to have succeeded, only to be thrown back into the sign on loop when I attempt to do anything.

Needless to say, I’m not able to sign on with the Media Player app either. I am able to web-access the server, using the same ID.

I’ve had to register a dummy account even to post this.

I’ve cleared all Plex-related content from my browser cache and cookies, delete all of Plex from my system (including Application Support and other normally hidden files) restarted my server (which runs on another host), and reinstalled Plex Media Player. All to no avail.

macOS: 10.13
Plex Media Player: 1.3.10.720-dfcd90a6
Plex Media Server: 1.9.4
Safari: 11.0