iOS 9.0.1 Unknown Error (-42585) What is it?
There are a whole raft of obscure error messages where your primary recourse is to contact Apple Support.

They have access to the engineering team that can look up a specific error message and they also have the best database of user issues that I'm aware of and can search for others with that situation.

You can also pay attention to if you're playing an Apple Music radio stream, or an iTunes match streamed song or a song that's copied onto the iOS device over USB from iTunes. Any raft of server errors like bad ssl certificates, missing files on Apple's servers would result in oddball error messages until the system can be updated with proper english errors like try playing another song.