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Control Panel - Info Center will tell you which version of DSM you are using. 
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It’s somewhat common for permissions to get mixed up when uploading. 
 – If you’re using Windows, when you upload to DSM, the folders are created and only allow your username to see them.
 – Linux is, by nature, strict on this (one user cannot see another’s files without expressly granted permission)
 – DSM doesn’t prevent or manage permissions very well.
 – It can happen, which it seems like you might have here, Permissions for Plex granted at the Shared Folder level, need to be manually extended again to all files and folder inside the shared folder.
Look here to see if this FAQ is of help. While fixing Plex’s permissions, you can also fix the permissions so you can continue to upload