Switched from HttpListener to TcpListener to make calls to the JSON service possible for other devices on the same LAN without the need of setting an urlacl.
The app needs an exception in the firewall to make that work. Just confirm when starting the app for the first time. You do not need to set this if you do not want to expose the endpoint to other devices.
I removed the possibility for setting an address manually since the TcPListener is listening to all IPs on the machine already.