warning: failed to load external entity "http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl" | cannot parse http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl
After a quick investigation I arrived to a conclusion that the problem actually resides withing the sourceforge servers, which are in a maintenance mode: The sourceforge.net website is temporarily in static offline mode. Only a very limited set of project pages are available until the main website returns to service.