Verify your main database file is in good health.
- activate debug logging (not ‘verbose’!)
- quit Plex Server
- wait 1 minute
- start Plex Server
- wait 5 minutes
- fetch log files and attach them here
Or inspect them yourself. Take a look at the Plex Media Server.log file and seek for messages about database corrupt or malformed.
If you find these, you may have to repair your database.
https://support.plex.tv/articles/201100678-repair-a-corrupt-database/
And standard question for Synology user: Have you done the changes described in here?
