When I try to move the mouse over the Label now, the mouse-picture gets caught at the edges of the Label. The mouse is moving on invisibly, emerging on the other side of the Label, and then the pictures changes its location to the new mouse location.
To make it short:
QLabel with normal text: setMouseTracking(true) works.
QLabel with richtext: setMouseTracking(true) dosn't work.

Whereas doesn't work means: compiles fine without warnings, but doesn't track the mouse while running.