Are you running DSM 7?
1.21.0.3744 is an early beta release and not fully compatible with the current version of DSM 7.
Download the current version from plex.tv and perform a manual update.
- Make sure you download the DSM 7 version of PMS.
- The current version will create a new shared folder, /PlexMediaServer, and move the Plex data folder to that location. The migration can take time depending on the amount of data to be moved (minutes for some, hours for others). Do not stop the process.
See DSM 7 - Packaging 2.0 (PMS 1.24.2.4973) - Release information for additional details.
The FAQ also has helpful information: Synology FAQ - Questions, Answers, and a few How-To’s
