A few things you likely noticed:
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By default, your username doesn’t see the Plex share. It’s there for debugging only.
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By default, placing media in the Plex share is both ill-advised (I have 6 warnings there each in a different language) and it drives PMS itself crazy (recursive scanning). Your performance will suffer terribly. If you have media in the Plex share, PLEASE REMOVE IT
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As of PMS 1.15.4.994, I removed user Plex from the adminstrators group. You must now manually (Control Panel - Shared Folders - Permissions tab for each of your media shares) give user Plex permission to read or read/write as you deem appropriate.