You can't bake AO and vertex color at the same time in xNormal. So yeah, make sure you tick Ignore Vertex Color on the high poly mesh and aren't baking vertex colours.
I have ran into baking AO problem when I was baking the high polys vert colors. When I rechecked the "Ignore per-vertex-color" box my AO would bake properly. Not sure if this is the same problem might want to post a little more info.