you must be careful here.
Backups of the Plex data from DSM 6 (databases & metadata) cannot be used in DSM 7 directly. I’m saying this to be clear as I’m a bit confused.
If you have only problems with MEDIA, there is no need to reinstall anything.
We just FIX IT 
This sounds like a permissions problem with the media files – which is easily corrected.
In this FAQ, this is how we reapply the permissions.
You must pay extra attention to step 11. where it shows Apply to this folder, sub-folders, and files
This will tell DSM to fix permissions for everything in that shared folder.
After it’s done, Have Plex Scan files again
