Forced account sign in and server could not be found

Thanks for the prompt reply.
Firstly, can I ask if the need to have the server and the app signed into plex.tv so that they can be authenticated effects the basic local operation of Plex, or not?
and if so, is this a recent change?

I’ve been using Plex on my local NAS for more than three years, without having to claim the server or log in to a Plex account on the server or app. This is the first time I have encountered mandatory log in to get it to operate.

I’ve been working through all your suggestions over the past few days, but I haven’t had any success yet.
I’ll keep trying and work through the steps in Q16, which is leading me to;
https://support.plex.tv/articles/204281528-why-am-i-locked-out-of-server-settings-and-how-do-i-get-in/
https://support.plex.tv/articles/204604227-why-can-t-the-plex-app-find-or-connect-to-my-plex-media-server/

I’ll report back when I’ve been through these processes.