When moving a server it’s best to follow the official procedure.
https://support.plex.tv/articles/201370363-move-an-install-to-another-system/
IIRC this was due to security issues where somebody who gained access to your Plex Media Server could point the AppData folder to a prepared folder to either compromise Plex or access system files.
From Plex Media Server - #338 by drzoidberg33
FIXES:
- …
- (Security) Remove the Windows AppData path preference from the web app (#11355)
You should be able to remove the old server from the list of authorized devices – otherwise it should go away after some time by itself.
https://app.plex.tv/desktop/#!/settings/devices/pms