Thank you very much for your help. This does work but Ido not to understand what is going on here under the hood.
1) Why would using "mdiArea->activeSubWindow()->pos()" work and "this->pos()" in the subwindow not?
2) Why does "this->pos()" work on a "regular" window work and on an Mdi subwindow not?
3) In the meantime I'm also struggling with the problem that my overwritten "moveEvent(QMoveEvent*)" method in the Mdi subwindow does not seem to get invoked when changing the position of the window.