At least on Android anyway, I don’t know if this is a client or a server problem.
The first message I was getting is, ‘Streaming Session doesn’t exist, or timed out’ when attempting to stream music from plex on Android (local or remote). I can however sync it to my phone (local or remote) and then play it.
I haven’t seen this first few message for a few days, so I assume Plex have fixed it an android update.
However, lately I also note I get the following error, which seems to be for synced items only:
Error, 'The connection to the server was lost. You can try again or browse this device instead. <Try Again / This Device>
Then I am forced to choose ‘This Device’, which takes me away from my server, which is quite annoying.
More broken features at release. 
