Just realized, that I had still thread still open.
I'll share how I managed to circumvent the problem. Sadly I haven't found a real solution as of now.
In the function that makes the screenshot, I added this line just before calling QWidget::render
this->setStyleSheet("cStackedWidget {background-color:white;}"
"QWidget#LiveStatus{background-color:white;}"
...
//List of all QStackedwidget Pages
...
"QWidget#FanThreshhold{background-color:white;}");
For some reason I have to set the stylesheet new for each time. Simply setting this once will not work.
Anyway, I'll mark it as solved,
unless someone else has a new idea !?