Scene goes white when Floor is set to invisible to camera

[butaixianran]

Need help, when "Foreground alpha" is checked and Floor is set to "invisible to camera", everything just goes white.

When "invisible to camera" of floor is unchecked:

This scene is set in C4D with 4 area light around the charactor. A key light, a fill light, 2 back light from left and right.

Why this happen? And how to fix it?

Thanks