I experienced the same error today on Plex Web with Firefox and the Windows 10 Plex Player desktop application along with the Error Code: s3015 (Media). I get that message for any media I try to play (I tried flac 44/16 and mkv H264 videos).
Chromium-based web browsers (tried Chromium and Edge) just seem to play the music but no sound…
Plexamp on Windows 10 just shuts down every time I try to play something.
On the mobile side (Plex Android app on LineageOS Android 7) it works fine.
As I’m running Plex from the Linux Server docker container I tried to revert to an older version of the server: didn’t work so I returned to the latest.
In the end I rebooted the computer as it seemed to be the only device to have issues and now it all works fine on Firefox and Plexamp Windows 10 !
I know the last answer is like 2 weeks old and you probably already tried the “switch off and switch on again” solution but for me it did the trick.
TL;DR - Reboot the player and the server
