Hardware accelerated transcoding is broken for the DS920+ and other systems with Gemini Lake CPUs (Celeron J4xxx). See: What kind of CPU does my Synology NAS have?
The current workaround is to fallback to 1.32.1.6999. Download: 1.32.2.7002 - #31 by ChuckPa.
Synology does not permit direct fallback to prior releases. You must uninstall the current version first. In Synology Package Center, choose Uninstall → Are You Sure → Uninstall Only. You can then install 1.32.1.6999.
Plex is aware of the situation and working on a fix. You can monitor the Plex Media Server announcement thread to be notified when a fix is available. Scroll to the bottom of the thread and change Normal to Watching.
