Only casts to Chromecast when server is not signed in to Plex account

[r4pt0r66]

Since a week or so ago (perhaps with the latest Plex update on 9th Nov, but I'm not 100% sure). Rolling back to previous Plex version does not resolve the problem:

Casting only works when the server isn't signed to a Plex account, and therefore only visible on the local LAN. I've built a new VM and redone the setup of the channel and replicated the issue.

Playback on mobile devices and browser still works over LAN and Internet.

Any one else? Has Plex thrown a spanner in the works?