Hi Ivan,
Finally, I got Plexamp to work on a fresh installed Pi with the help of your instructions and server.prod.js. However, it is not your version hacked version that is running in the end because I ran into MPD errors as well. Neither localhost nor 127.0.0.1 worked.
I switched back and forth between your hacked version and the original. Your version worked to retrieve all the required connections data on the pi (the resources-server.json). The original version now is playing perfectly.
At last I am very happy. 
Thanks 
I was already switching over to Jellyfin and Jellycli but gave it a last shot this morning, starting over from scratch. Because I love Plex’ radios and I am a long time Plex Pass user.
It’s an absolute shame that it is this difficult these days to use Plexamp as headless audio player connect to a decent HiFi setup.
(it’s a Pi 3B with HifiBerry DAC+ on Raspios/Raspian Buster by the way. Tried Bullseye as well but you cannot get NodeJS v9 on Bullseye anymore from the nodesource.com repository. It was necessary to use xkonni’s instructions as well)
