See if there is another app running on that computer which is occupying port 32400.
Usual culprits are Torrent clients or ftp servers.
Second possibility: If the Plex data folder has been relocated previously, its location might still be contained in the registry, but has been rendered invalid. (For instance due to changing folder names or drive letters.)
Check the registry key mentioned in this post: [HowTo] An extended guide on how to move the Plex data folder on Windows
If it exists, look at it’s content and verify that this storage location actually exists.
