Did you recently update the firmware of the NAS from DSM6 to DSM7?
This is a rather large change for any app running on the NAS. Certainly for Plex.
Item #26 on the Synology FAQ deals with this specifically: Synology FAQ - Questions, Answers, and a few How-To's
Unfortunately I have not a Synology NAS myself, so my ability to advise is limited.
