Rendering artifacts of QLabel inside QGraphicsscene

After changing my ClearType-settings in Windows 7 once, I started getting render artifacts from QLabels that are included via QProxyWidgets inside a QGraphicsscene. The error applies to every QLabel that is part of a QGraphicsscene, but not others.
I resetted my ClearType-settings to default, but the error persists. It only seems to occur on my system though, on other machines the text looks fine.