I run Plex on Ubuntu and a Synology NAS. My knowledge of Plex on QNAP is limited.
Creating a PlexData share is covered in QNAP FAQ #28: How to use the PlexData share. There is additional information in this thread.
If I’m reading it correctly, the folder is a diagnostic aid. Changing permissions would not delete any data.
My suggestion is to
- Stop Plex Media Server.
- Revert the permissions you changed. Make sure the desired user(s) have RW access to the PlexData folder.
- Then restart Plex Media Server.
Hopefully that gets you back to where you can see the data.