I changed nothing, then clicked the render button again. It looked a little better the 2nd render. The bottom stair still looks a bit off though. Also, it lit up the stairs erratically when I rendered a 3rd time.
Are you sure it’s an error? It looks to me like the horizontal surface of the sidewalk is reflecting light in to fill the shadow area on the risers. I would expect to see that in a real scene if I was photographing it.
I would check the material properties of the top 2 steps (horizontal surfaces) as I think they may have an “emitter” (light) property? I would also display your model with the monochrome setting (so you can see if your “faces” are oriented consistently, as an applied texture on a back face may look correct inside sketchup, but may not render as expected inside your renderer.
is that top screenshot from inside of SU displayed in “monochrome” as the brown to the left suggests it might not be? just to verify face orientation. unless you have rear faces set to a brown color?
good you deleted some of the interior geometry as sometimes there can be light leaks from that.
when errors are intermittent like that it does make it more tricky to diagnose