Would be nice if we could get it working with using another user-agent.

Beta of Domoticz use already a other user-agent. ;)

Just installed the latest beta (Version: 3.7450 build: 2017-05-11 18:05:12), but Toon still fails to connect:
...
2017-05-17 16:25:06.012 ToonThermostat: Worker started ...