BrowserHawk uses the DOM internally. Sometimes the version of the DOM used by BrowserHawk conflicts with the version used by your server. Each server has different class loader behaviors, so there's no guaranteed solution that works for everyone. One thing to try is moving bhawk4j.jar to different locations (try WEB-INF/classes and your server's classpath directory). Another thing to try is unjar'ing bhawk4j.jar and rejar'ing without the org.w3c.dom package.