OK, thanks
. It would have been my next step to move in-and-out .kde4 folders anyways to see what stops me from starting up.
But I still ask others who might be more experienced with kbuildsycoca4 (I admit, I have never realised it even exists before) to give tipps about possible problems (as I am almost sure, that 99% of my configs etc. are not the problem ... but finding the 1% might get hard
)