Cec, this is your file, just opened and directly rendered:

...as you said, no problem.

But zoomed in only a little bit:

... the problem reappears.

The biggest floor is a little bit displaced, so I tested if this can be the problem. But neither push it to the right position, nor delete the floor and rebuild it helps.
If you create a second level and then increase the original level by 1mm, you can see strange "rays" on the floors in the 3d view:

I suspect this has something to do with the problem, but I still can not figure out what exactly caused it.