Also, are you running DSM 7.x on your Synology?
If so, you should update to a later release.
1.21.0-3744 was a beta release meant to support beta DSM 7. It is no longer supported and not fully compatible with DSM 7 production releases. Unfortunately, Synology will not update or remove it from their systems.
Download the current DSM 7, Intel 64-bit version public or Plex Pass release release from plex.tv (the AMD CPU uses the Intel 64-bit instruction set): https://www.plex.tv/media-server-downloads/#plex-media-server.
Perform a manual installation via Synology Package Center.
This will create a new shared folder, PlexMediaServer, where the Plex Data Folder will be located. The folder exists in a non-user accessible location when running 1.21.0-3744.
Depending on the size of your libraries, it may take some time to relocate the Plex Data Folder information. Do not stop the process, even if your DSM session times out.
Some additional helpful documents:
Synology FAQ - Questions, Answers, and a few How-To’s
Installation & Setup of Plex Media Server on Synology
