I suspected is was some stupid little thing I missed. Thank you!

Yes it's at:

http://www.mediafire.com/download/4pgzn ... Floor.sh3d

I have a basement that is smaller the level above. The room above the basement has no floor. I see there was a defect that was fixed in 4.3

"Fixed a bug that prevented to display correctly some floor rooms in the 3D view when they overlap."

but I'm using version 4.4 with (Java 1.6.0_45 / 64 bit ...