I’ve been having this error myself lately, apparently it has something to do with corrupted files being scanned somewhere in the library. Do you by any chance have a music library? This Plex forum user was having the same troubles. I noticed when I added music folders to Plex that were once controlled by iTunes I started getting that error nightly.
Check that thread as it may have a solution for you, and if not let us know and hopefully someone else can chime in because I am no help with troubleshooting 
