I found the problem all by myself
The RenderAccel option in xorg.conf is the source of the error. Disabling it makes kde work fine again.
But without rendering acceleration, composite features are unusable, it makes kde soooooo slow, you have time to get yourself a coffee while konqueror opens...
It's appearently a known bug, I found traces of it in nvidia's buglists. Let's hope that the next driver version will fix the thing.