It came up on the Debian bugtracker that the search box of the build-in documentation tries to use a search service at http://localhost/nokiasearch/GetDataServlet (when viewed over a local web server, when viewed from the file system, the URL is even invalid), which does obviously not exist. Since the documentation is meant for offline use anyway, it rather makes sense to build it with the offline doc template, as seen in the attached patch from Debian.

Note that Qt 5 already uses the offline doc template, so no action is needed for Qt 5.