Register user broken (but tried the obvious...)

Hi, My user registration screen is broken and returns a -1 error whenever I try to register a new user. It is on my own xwiki server, not on the farm. I've tried enabling the superadmin user, editing, and re-saving the RegisterNewUser document but I still get the same problem. I have two wikis running on this server, with 'virtual' switched on in xwiki.cfg; the registration works fine in the default wiki but not in the new one I have created. I'm an utter newbie to xwiki so it's possible I'm missing something obvious... if someone could post step-by-step instructions as to what is normally done to solve this problem, I'd be grateful. Many thanks Jonathan Essex.