On the material panel, uncheck the “ZInvert” button, for the red material, and it should work.
Edit : ooops, I forgot : also uncheck the “Env” button, on the “mirror transp” tab (still on the material panel).
Thanks a lot duskblue. I must have turned those on by accident.
I really couldn’t find out what went wrong, I thought it had something to do with the render settings of the scene.
Thanks again.
For those who were wondering what was wrong: have a look at the following two images.