You should never use “run as Admin” with Plex server. It can mess up the access permissions in your Plex data folder and thus create more weird behaviour.
At this point, the best I can recommend is to perform a thorough uninstallation according to https://support.plex.tv/articles/201941078-uninstall-plex-media-server/
Verify that Windows has classified the network connection as “Private”, not as “Public”.
Continue with
- Make sure that both your Plex server machine and the machine where the Plex web app is open have precise date and time set. Activate automatic time synchronization.
- disable any 3rd-party web browser add-ins temporarily
- investigate if your router is applying “DNS rebinding protection” and insert an exception from this protection for the domain name
plex.direct(if possible)
(background information: https://support.plex.tv/articles/206225077-how-to-use-secure-server-connections/ ) - verify that your local network is indeed using an IP address range that is reserved for “private” use Private network - Wikipedia
If you are still unable to acces server settings or to create libraries, perform Method 2.2 from this post: You need to reclaim your server, but don't dare to touch the Windows Registry
