After playing around with Qt Creator, the mouse focus went crazy.
The focus for the mouse was for example on the application behind the most front window. Very strange. After some seraching for the problem I came to this forum threads, which describe exactly my problem:
In the second one, someone wrote, that his Logitech mouse lead to the problem. Same here. When I unplug my Logitech Mouse receiver, then every thing works fine with the mousepad/trackpoint. When I plug it in again, the problem is there again!
For now I haven't found a direct solution to the problem - just a workaround.
It seems, that rebooting with the mouse receiver unpluged solved the problem.
I can use my Logitech mouse again now.