Forget what i said. Just forgot that the transformation needs to be installed before it can be used 
I’m guessing you’ve not used JSONPATH before. This is an installable add-on.
Note also,particularly when making edits in Things files, many bindings are not always effective at picking upchanges in flight. Sometimes a binding restart is needed.
Thx for the hints 
So my from my above mentioned problems 2/3 are solved (Transformation works and rule triggers) and the base problem (Transport to OpenHab) is solved also.
Because the third problem (OpenHab not starting automatically anymore) has nothing to do with the thread title, I will open a new one.
Many thanks to all helpers !!
(And I promise to further read the docs…)