I look at Plex.tv. It has record of PMS 1.40.4.8840
This tells me that the Syno has not logged into Plex.tv
Have you attempted to:
Uninstall the package (KEEP option; first radio button)
Reinstall the same SPK – HOWEVER
– Use the “Plex Claim Token” radio button
– Click the “Get Plex Claim Token” hyperlink
– COPY the token from the browser tab
– PASTE it into the install page
– Click NEXT - NEXT - DONE and let it install