Imagine my shock after months and months of no updates for Thecus when I clicked on the Manual Update link tonight expecting no compatible updates like many times before, and received PlexMediaServer-1.25.2.5319-c43dc0277-x64 instead.
Last I had heard Thecus wasn’t updating their host OS with libraries that newer Plex versions needed, and Plex wasn’t building anything compatible with the older systems. So no upgrade possible past PlexMediaServer_1.14.1.5488-cc260c476_x64
Took a chance and ran the upgrade from PlexMediaServer_1.14.1.5488-cc260c476_x64 to the new version and IT WORKED!!! Fast and painless- not words that I would normally say together based on my past history with Plex upgrades on this particular NAS.
I don’t expect anyone other than me to be excited by this as I expect everyone else has probably moved away from Thecus. I still have an N7770-10G going strong- even though it’s no longer my primary Plex Server I try to maintain it in case of issues with my Primary Windows Plex Server.
But if there still are others out there interested- you may want to give it a try. YMMV- but mine worked. My N7770-10G is happily chewing through metadata updates and other library upgrades it couldn’t do prior.
Thank you Plex for an early Christmas present 
Perfect timing as my Nvidia Shield is FUBAR following the Shield 9 update which has broken Plex.