My suspicion is that the passed values to instantiate_and_mount are incorrect, or you have some half-broken entries from some earlier attempts. See xowiki/www/admin/test.tcl for an example where a new instance is created using "site_node::instantiate_and_mount".
For proper subclassing xowiki (to build some own value added versions of xowiki) see the s5 package in the openacs cvs.
