First thing I’d do is to verify that not another instance of Plex Server is already running.
Thus can happen if you have more than one user accounts in Windows.
So, log in to every Windows user account and explicitly log off afterwards.
Don’t do a ‘Switch User’ action, this keeps the user logged in.
Next thing to do is to quit and then uninstall Plex Server.
(Use the regular Windows ‘Apps’ control panel - don’t use 3rd party ‘Uninstaller’ software!)
Then download a fresh copy of the most recent installer from the ‘DOWNLOADS’ link above.
Maybe use a different web browser than you normally use.
Install it,
but decline the offer to start Plex at the end of the installation process.
Reboot the machine and let it start up. Give it several minutes to finish startup.
Then try again. Make sure the machine has a working internet connection, at least the first few times you use Plex.
