This bug is caused by a Rhino bug in V6 (CRhinoGet::PostCustomWindowsMessage API). Anyway, since it may take a long time for Rhino to fix it, so we do extra development to avoid that API inside XNurbs. The following is XNurbs V1.21. Replace the existing XNurbsRhino.rhp with this new rhp. Let us know if you are happy with it.
XNurbsRhino.rhp (311 KB)