Have you tried restarting the Plex service?
Also, maybe check permissions on the folder for the library?
Check out Chuckpa’s tool for database maintenance, maybe the db is corrupted?
I know this Reddit thread isn’t specific to Linux, but Synology runs on Linux technically.
https://www.reddit.com/r/PleX/comments/10z7ng5/your_changes_could_not_be_saved_issue_while/
