I think your problem is identified in Archicadwiki here -
http://www.archicadwiki.com/TechNotes/Intersection_of_curved_slanted_and_complex_walls_in_2D?highlight=%28TechNotes/%29%7C%28%E2%80%A2+ArchiCAD+11%29
They suggest using the patch tool.
Sorry, it's not a lot of help to you but at least they have identified it's a problem!