Your webservice does not deliver a valid ARML file, or does not deliver Content Providers at all. Please check the response of your webservice.
Could you check if there's really a problem with the ARML or is this still an ongoing problem? I'm using the PowerHour classes to build the document. (I also got this error when trying to create a world using the url of the world I previously had help getting uploaded - see post at http://devzone.wikitude.com/web/forum/home/-/message_boards/message/16315?p_p_auth=fr5y2fR6)
alex