I also have the same issue - shows and media suddenly not playing anymore that were playing fine last week. (FireTV 4k)
I just downgraded to Plex for Android 7.27.1 and this is working fine again.
For anyone else needing a temporary fix until new release.
Download APK from here Plex for Android - #331 by SamFromPlex
and then use Apps2Fire to sideload.
