I got my 535 back from service. They replaced the touch panel due to "malfunktion of electronic module"
It now scrolls far better, if a finger has contact to the screen, but not 100% smooth. Faulty insertions ("accidential clicking when scrolling") seem to have been eliminated. Long clicks (for context menues or changing the order of the tiles) are now beeing registred fine every time.
The screen does does not feel that sluggish as before the exchange of the touch panel.
Pinch to zoom is still not that good, because the touchpoints have to be placed comparatively far from each other, to be registred as two, but it is working satisfiable.
Multitouch of three fingers is not working properly, four or more simultaneous touchpoints are not beeing registred.
Doube tap to wake is working quite well now.
I strongly recommend getting your 535 to service.