When I set both flags to false, I had an ugly rendering result (but the flickering stopped).
Now I found a solution for me:
I set the rendering rectangle to zero:
SExposedVideoData videoData = SExposedVideoData(0);
rect<s32> rec = rect<s32>(0,0,0,0);
driver->beginScene(true, true, backgroundColor,videoData,&rec);
Now my window is not cleared anymore and my texture looks like it should...
Thanks to all...