Hi Jürgen
First of all thank you for your help, things are working now (thanks to you), but now I would really like to understand what I’m doing 
On this page MQTT Arrives in the Modern openHAB 2.x Architecture | openHAB the thing-syntax is described with a colon (and a mqtt: in the beginning) Thing mqtt:topic:mything and you have spaces between Thing topic sonoff_1_Thing - do you know if both are valid, or is it just a typo on the blog-page?
Your example has Light_1" @ “Sonoff”, which the example on the doc page doesn’t have MQTT Things and Channels - Bindings | openHAB - what is the purpose of Light_1" @ “Sonoff” ?
And last but not not least.. the channel part of the item is a “little” confusing - it looks like its a mix something from “bridge”:“thing”:“bridge”:“thing”:“channel” - i can’t find any documentation about channels in general. Do you know if its described somewhere, or can you explain it ?
I have tried to visualize it with this image
