Hey @zubredo, sorry for waking up the dead 
But I wanted to thank you for your help!
It did not work for me but brought me to the right solution. Just in case anybody has the same issues as me:
When I run the curl -X GET -k -i 'https://dresden-light.appspot.com/discover' I got the the following result:
[{"id":"XXX","internalipaddress":"<IP>","macaddress":"XXXX","internalport":<PORT>,"name":"XXX","publicipaddress":"XXX"}]
But I could not access the IP and port, I needed to ssh into my hassio system. Look here!
After that, I needed to unlock the Gateway under the deconz Web UI > Setting > Gateway > Advanced > Connect App and then I could finally access the REST API from deconz with:
curl -X POST -i 'http://<IP>:<PORT>/api' --data '{"devicetype": "my application"}'