To be able to go back running as a regular user, you need to reset the access permissions of the plex data folder. Of every folder, subfolder, and all files therein to default.
Here is a tool which makes this a bit easier than the default Windows tools: Resetting NTFS files permission in Windows – Graphical Utility – lallous' lab
(do it while plex server is shut down!)
Now to fix the other issue, you may need to perform this: You need to reclaim your server, but don't dare to touch the Windows Registry
Be aware of this afterwards: New Claiming Requirement for Windows
