The SONOS API changed.
SONOS removed the "Endpoint URL" form field from the screen that you use to add the custom music service:
http://your_sonos_controller:1400/customsd.htmNow, as of SONOS version:
Version: 15.8
Build: 75145120
Sonos OS: S2
The only option is "Secure Endpoint URL".

So, any other service that used the "Endpoint URL" in their code will not work until a code change is made. Example, Airsonic.
Sadly, Sindre has abandoned pretty anyone who paid for the lifetime license. It sucks because I was using Subsonic since 2015 and I am the genesis for Subsonic SONOS integration. I contacted Sindre back in January 2015 asking if he could look into the SONOS API. I then spent time working with him testing his new software on my SONOS system. By March 2015, there was a final version of Subsonic that supported SONOS. I still have the emails as I've tried contacting Sindre numerous times. It's disappointing that he has not open sourced this project for others to carry on.