Spaces are not allowed in URL's - they are meant to be escaped to the %20 format.
In Stanza are you using the option to set up Calibre as a Web Page or as a Catalog? I get best results using the Catalog option - and that means that the URL is the of the form:
whereas if doing a Web page you leave of the 'stanza' part.
I have had no problem using that way of working with Stanza.