There’s a pinned post, DSM 7.0-41882 "Release Candidate" breaks Plex - Do Not Upgrade , about a latest release breaking synology devices. Might be related to your problem.
The only thing I can say that fixed my Android devices is installing the plex server application to my windows desktop and reroute everything through my computer. I wasn’t able to update the very outdated plex server on my WD MyCloud Home drive, because WD pushed those updates when they thought it was time. Not sure why I didn’t look into that before as my computer is more powerful then the device I bought.
