Thanks for the screenshots.
Your old server, from when the NAS ran DSM 6, is still there, in the /Plex folder.
It is possible to recover that server.
The process restores your Plex server to the state it existed on June 11 (the db file date in the screenshot you provided).
The current (new) server will be wiped out (just the server, media files are not touched).
Media files added since June 11 will be rediscovered. However, any customizations (collections, metadata edits, etc.) made since June 11 will be lost.
It is not possible to merge the two servers.
To proceed with restoring the old server, see this post: Struggling with migration from DSM 6 to DSM7 - #2 by FordGuy61
Read it completely before starting the process and write back with any questions.
Note 1: The process involves uninstalling and installing Plex Media Server. When re-installing PMS, use either the public or beta version from plex.tv.
Note 2: Before starting the process, enable Sync Watch State & Ratings on your account. Have anyone you share with enable it as well (it is an account, not server, setting). This will save the watch state for any media watched since June 11.
