Hardware decoding in Windows is only allowed to apps which run in the user context. Which means that if you run Plex as a system service, you cannot use hardware decoding.
You must use “automatic user logon” after bootup and then start Plex either automatically or via the Task Scheduler.
Another potential wrinkle is Remote Desktop using the hardware transcoder itself as long as a remote session is in progress. See Windows Remote Desktop breaks HW Transcoding (QSV) - Here's the fix!
