@agocs hmmm, I tried doing that, inserting makeCurrent(), but that didn't change anything. Still get a black screen.

btw, when I deactivate drawOffScreen(), only calling drawScreen() it gives me scrambled data (undefined) on the screen, where I would expect to see a white quad, because of glColor4f(1.0f, 1.0f, 1.0f, 1.0f); in drawScreen()