I don't know how you're serving your instance of audiobook shelf, but If you're using nginx you need to Upgrade the connection to websockets.

Yeah. I have it on docker on my synology but the infrastructure to get it from my synology to my phone doesn't work. I've tried tailscale, ddns, nginx with an external website... It just doesn't connect.

Whatever your set up is, I'd be grateful for any pointers.

Used to be a dev but no time to fiddle now with kids.