Flamingo 5.5 - rendering background problem

I am trying to do something really simple: render on a white background, using no lights, just the backdrop set to white, all set to white.
It is not working.
I not![flamingo_pass1iced, though, that the fist rendering pass renders as it should, while from the second pass I get a standard grey that I see nowhere in my settings.
Of course pulling the backdrop I get the white, but lose all the shadows.
Any idea?