What do you mean by "multiple polygons"? Do you mean any object with more than one polygon, or something else?

The error seems pretty definite: Something about the objects that you are using is causing the XSI's boolean code to fail.

In modeling, Boolean operations are those that add, subtract, and mask shapes together to produce new shapes.

Yeah, you're probably going to want to get rid of the other meshes, or extract them into their own objects, and then delete the original.

The way to go about that would be to select a polygon on say, a collision sphere. Under the select menu, hit grow selection a few times, until you have all of the polygons in the sphere selected. Then, go create->Poly. Mesh-> Extract(delete). This will pull the selected polygons into a new object, and delete them from the source object.

Once you have the visible geometry in an object by itself, you should be able to perform boolean operations on it.

No problem. To be honest, I was just guessing.