What version of PMS do you have running on your Shield?
I understand the PMS Shield version isn't on the play store, however I was having " Connection to the server was lost" problems (annoying as the server and app are BOTH on Shield), so ended up deleting Data, getting stuck, uninstalling the software, then re-installing an APK i found.
So probably not running the correct version.
The Plex Media Server 18.104.22.16809-42bd8c63f
The Plex for Android 22.214.171.1242
If this is the wrong version, is there a way to reset / reinstall?