http://pd4ml.com/tlds/pd4ml/2.6 in the context is not an URL, but an unique identifier of a JSP tag library (pd4ml_tl.jar).

If the JAR is correctly added to your web application it “registers” itself among application resources under the name.

If your application tries to unsuccessfully connect the URL, that means there is a webapp misconfiguration.

I would recommend to download the most recent version of PD4ML, install it and to make sure there is no old cached instances of pd4ml.jar in the working dir of the application etc.