Please follow the instructions from @sa2000 one message up; it will help Plex with any root cause investigation.
Also please save a database backup in case it is required or requested by @sa2000. 
Check out this post (below) for a generic shotgun/cleanup script that should be effective against that issue. The SQL should clean up those items and their related entries.
Slow Query time - unable to scan or play media - #37 by jakehoms
