Have a simluar issue like the one you have fixed.
The Plugin can't connect to my Offical Plex Dokker hostet on the same Unraid Server.
On the Dashboard i got the "Please make sure you have setup the plugin first" Message and under Setting the Plug in won't find any server. I confirmed the Login process and stuff got a Token but no work.
Using 6.9.1 but on 6.8.3? it doesn't work either