This looks like a plane from a bug with the 3d interactive widgets (see Enabling ray tracing with glyphs). This also looks like what is described here : OSPray pathtracer: erratic walls.
Is there any filter that creates an interactive 3d widget in your pipeline (like the plane of the Clip filter) ?
If so please try these steps :
- open your scene (do not activate the ray tracing before, not even once)
- toggle off the visibility for all of your interactive 3d widgets
- now toggle on ray tracing