[Stiglr]

I understand now.

I've had that problem, too, with poly lines leaving extra vertices (and a telltale white line) after you've changed the poly into a 3D object.

Not sure if this will be the answer, but check and see if there are any double vertices along the line you drew the original poly line, or even if the poly itself is an additional object from the extruded object (isolate the object from the poly using Hide Unselected commands, etc.). Try to Merge them, or failing that, weld together any overlapping double vertices; in effect, make the poly line and the object you extruded from it ONE object with no double vertices. Maybe then the Boolean will work on it.