Rene, sorry you are still struggling with this issue....
If anyone on TM knows the following method I am suggesting can create issues with this type of engine, please SHOUT OUT... any criticism welcomed.
Rene an old fashioned method to check if a fault might be fuel related was to use an aerosol can of EASY START... I suspect there will be a German equivalent. The can contains a HIGHLY flammable fluid that sprays out as a vapour. it was often used to fire up reluctant engines, in cold weather or can be used to test if a non start issue might be caused by lack of fuel....
The usual method was to remove the air filter and spray into the engine air intake for no more than a second or so, while someone else turns the engine over..... if it tries to fire up and run on a little, then you may have a fuel related issue.... most likely electrical...?
Spraying too much risks a backfire thus CARE is required, no smoking or naked lights while using the contents of the can...
I have heard of brake cleaner being used for the same task, but never tried it myself...
