Tomcat 4.1 is pretty old... it's probably a problem with either the JVM version that's running this or just the Tomcat version. It might also be a problem with the version of the Xerces XML libraries not working with older JVM code.
I'd suggest upgrading to modern versions of both the JVM and Tomcat (at least JVM 5.0 and TC 5.5). In theory, Jforum should run on older versions but this hasn't been tested recently.
FWIW, it looks like it's trying to parse a local file name (C:\...) as a URL for some reason.
[originally posted on jforum.net by monroe]