Hi all
I created an account just to share with you my findings.
I think the OpenGL driver of the Raspberry Pi 4 is not playing well with SweetHome3D. I find that by modifying these lines in "/usr/share/sweethome3d/sweethome3d.sh":

The black boxes problem is gone. But I think it will use CPU to render so it's pretty slow, but is still tolerable to me [:)] .