Don’t panic.
It should be Migration.log
That said, ALL of your server’s metadata is now in the new PlexMediaServer shared folder.
DSM 7 forced a complete rewrite and restructure of everything.
Plex share → PlexMediaServer share – for metadata only (NO MEDIA)
Plex user → PlexMediaServer service
I would first like you to look at this. If I have surmised correctly, all you now need to do is grant permission for “PlexMediaServer” to see your media.
Here’s the How-To.