Workaround: Disable secure communications in Plex.
Plex Media Server: Settings → Network → Secure Connections = Disabled
Plex Android app: Settings → Advanced → Allow Insecure Connections = Always
Note 1: You will lose Plex Web access via app.plex.tv, which uses https, when you make this change. You will have to connect to your Plex server via http://nas_ip_address:32400/web.
Note 2: This affects all clients. You will have to allow insecure connections on non-Android clients as well.
