Thanks for your help, after playing a little with all the options, I've solved the problem. I'm posting the solution here, just in case someone else has the same problem: When you render a movie (not an image), the compression type used by default is "animation". You gotta choose a different compression type instead and render. After that, doesn't matter which compressor you choose, the background renders perfectly white (even if you pick "animation" again). I would like to point that we work with several copies of Poser, and in all cases the problem was the same, and this was the solution. Why the background renders black instead of white unless you choose a different compressor first, seems to be some bug of Poser.