in fact not an URL “to go”, but a unique ID for PD4ML’s TLD document. The ID is also present in the meta data of pd4ml_tl(_demo).jar. In a normal situation Servlet/JSP container discovers the IDs and do not even try to load http://pd4ml.com/tlds/pd4ml/2.6 as a web resource.

So in your case I would suspect problems with web application deployment.