If anyone is setting up their NVIDIA Shield as a Plex Media Server right now, be sure to go into the app settings on the device and roll back any updates. People are having issues where the update prevents the library from scanning video files. If possible, disable automatic updates.
EDIT: Before (or immediately after) rolling your Plex Media Server app update back, you will want to turn auto-updates off. To disable automatic updates, go to the Shield's Google Play Store, to Settings, to Auto-update apps, and disable the automatic updates.
Yup, broke for me as well - there's a Python error in the parser and won't scan anything on Shield TV.