Ok, this was my error:
I have several pages, which I used to show making them visible, setting the attribute Visible true/false.
Obviously this is not sufficient. It seems, that there are some invisible objects in the forground, which block the touch handler of a visible made page. Though, it's not really clear, why this behaviour is different, when I make the page visible for the second time.
If I call RestackTop() after making the page visible, all touch handlers work as expected, also for the first call of the page.