The problem was (supposedly) fixed in February in a 1.18.7.x release.
Make sure you are running a current version of Plex Media Server. Check Authorized Devices to verify installed version.
Version 1.19.1.2645 is available via plex.tv/download. You want the Intel 64-bit package.
The Synology FAQ has information on manually installing packages (versus using the version available in the Package Manager). See FAQ 3a.
The Plex Media Server Announcement thread has information on the latest releases. To be notified of a new release, subscribe to thread. Scroll to the bottom of the thread and change “Normal” to “Following.”
If you’ve never run Plex on a Synology, suggest you read Installation & Setup of Plex Media Server on Synology and the Synology FAQ.
Note the portions regarding media location. Do not put your files in /Plex or in Synology’s pre-defined media folders (used by their own applications). Doing so will cause problems down the road.
