In case you didn’t already try that:
Try to reinstall Plex Server following the steps from here: Synology FAQ (DSM 5 & 6) - Questions, Answers, and a few How-To's - #14 by ChuckPa
Looks like not properly uninstalling beforehand caused some crash issues with the latest updates, uninstalling helped one user here, I also followed his advice and had no crashes since.
