Ensure that your web browser is allowing app.plex.tv and 127.0.0.1 to access the local network: Important note about the Plex Web app - Local Network Access
Did you create a backup of the Plex registry branch before nuking W10?
If not, the server is not claimed, and has a new “identity” which makes all sidebar and home page links in your Plex clients invalid.
You need to claim your server (i.e. connect it to your plex.tv user account) again.
New Claiming Requirement for Windows
If plain claiming doesn’t work, you need to perform this procedure: You need to reclaim your server, but don't dare to touch the Windows Registry
