Render individual QImage(s) or QVideoFrame(s) in Widget without blanking QMainWindow

Hi Qt video specialists (and others),

I am in the process of upgrading my application from Qt 4.2 to Qt 5.5 and I am experiencing a problem. This problem concerns the way in which the QmainWindow and contained widgets are painted.

The task I have to perform is this. I cyclically receive a single video-frame from a 3rd party source. The frame is received in a thread that was spawn from the QMainWindow. On the QMainWindow lies a QFrame, of which the HANDLE is sent to the receiving thread. When a new frame arrives, it is (rather) simply drawn onto the QLabel that was added to the QFrame. In 4.2 this worked fine...

With Qt 5 however, the entire QMainWindow goes white when this is done.

So now I am trying to fix it and I think I will have to do some smart things with QVideoWidget, QAbstractVideoSurface and QVideoFrame(s). But I can't really crack it, so maybe someone here can help me out?
I found an example on this subject (for Qt 4.6). What puzzles me, is that the Qt5 example lacks the QAbstractVideoSurface stuff...

So basically the question is this: how can I render individual frames in a widget, without my main window going white? All help is appreciated!