I had a similar problem once on a Synology DS415+.
The server was not logged into the account and only allowed local access on localhost.
I used a SSH tunnel to use the Browser on the PC as if it were running on the NAS / localhost to assign the server to the plex account. After that it was showing up correctly again.
Shell (user probably needs admin/root access)
ssh -L 9000:127.0.0.1:32400 user@plex-server
Browser at PC to localhost:9000
