For some reason, attribute::exists_p seems not defined in your system, which is strange. Without being able to query attributes, xotcl-core fails to create its datatypes, so the missing definition of "PageInstance" from your excerpt is a consequence (such as many other undefined classes).
You should figure out, why attribute::exists_p is not available in your system. attribute::exists_p is defined by acs_subsite.
Do you have acs_subsite installed? if yes, what version of OpenACS do you have installed? What happens if you
search for attribute::exists_p in the api-browser?
