IIRC when you want to mix both, you need to enclose your OpenGL primitives call between painter-> beginNativePainting() and painter->endNativePainting()
Thank you for your answere. I've already tried that but it wasn't the solution. I've figured out that the problem occures when turning on OpenGL rendering. I guess it has to do with my hibryd graphic system. My main card has some serious problems so I can't run Optirun right now and I guess my secondary GPU does not support the required features (only OpenGL 3.X).
I'm not shure about that but as I said: turning off the OpenGL rendering solved that problem for me. Maybe it helps others.