Error 501 streaming my own music from Bubble upnp

Hi all,
I have Volumio on a pi4 all working great with a Kansas Tone Board DAC and sounding wonderful with flacs from my nas, Spotify and Tidal.
But I also produce my own electronic music and I like to hear how it sounds on various systems, so I save mp3s or wavs to Google drive, then use Bubble upnp on my android phone to download the tracks to the phone and stream them. This works great with a Chromecast, but when I try and stream to my Volumio device I get an error 501 on bubble upnp. Both devices are on the same WiFi. Can anyone suggest what might be the issue here?

Here’s a screenshot if that helps

Same here: Volumio works fine on Pi4+IQAudIO DAC Plus. I like the sound very much.
But, when I try to stream Tidal over UPnP, I get the same error message as you do, although the BubbleUPnP recognizes the Pi as a renderer.
I’ve posted this in a separate topic a while ago, but there were no responses from Community.