As described in the last post in the link, if I play a movie first the error message goes away.
Playing a movie every time I want to make a volume change on my RasPlex is brutal on the UX. To the point that I installed Emby last weekend and bought an Emby Connect account. Turns out I prefer Plex. But the issue with RasPlex volume control might still force me to switch.
Wondering if Plex personnel could chime in with any additional info on this issue.
Considering RasPlex is not an official Plex client, I’m not sure what info we can add. You might want to try asking the RasPlex devs. https://forums.plex.tv/categories/rasplex