Nvidia Shield: There is a very long thread on the topic: Nvidia Shield Dolby TrueHD playback is borked
The thread started with TrueHD problems but expanded to cover other Shield TV issues.
TL;DR: Fall back to 8.30.2 or use the 9.1.latest beta release.
Instructions to sign up for beta releases are here.
If you prefer to sideload, you can find the 8.30.2 release in the Android TV announcement thread and the latest beta in the Android TV Beta Release Announcement thread. For the Shield Pro, use the Google Play (arm64v8a) version.
For additional information on sideloading, see this thread.
Plex Web: For playback problems with Plex Web, try enabling the alternate streaming protocol.
Go to Settings → Debug → Show Advanced. Check the box next to Use alternate streaming protocol for video playback and Save Changes.
