Stop Plex server using its task tray icon.
Update to the latest Plex server version for Windows. https://www.plex.tv/media-server-downloads/#plex-media-server
If you have more than one Windows user account on this computer, log out from all user accounts. Then reboot it, and log into the user which you are using regularly.
Inspect the network connection in Windows. It must be marked as “private”, not as “public”.
Follow the steps in here: You need to reclaim your server, but don't dare to touch the Windows Registry
Then reclaim your server by following this: New Claiming Requirement for Windows
Ensure that your default web browser doesn’t have any browser add-ins, which block and suppress parts of web pages.
