I followed these instructions and it worked. thank you.
I stopped the docker container
I found the preferences.xml file under \UNRAID IP ADDRESS\applications\plexmediaserver\Library\Application Support\Plex Media Server\preferences.xml
I made a backup copy, deleted the keys:
PlexOnlineHome="1" (note: I didn't see this one in the file)
PlexOnlineMail="
[email protected]"
PlexOnlineToken="RanDoMHexIDecIALtoKeNheRE"
PlexOnlineUsername="ExampleUser"
restarted the server
ran the curl command as above (with the new claim code)
All seems ok now (i.e. the red/white exclamations seem to have disappeared)