When trying to generate the document, if your doc contains SVG, it throws a NoClassDefFoundError, because of a batik class which it cannot instanciate.
NoClassDefFoundError is a problem with your classpath setup. A Batik jar is distributed with FOP, make sure it is in the classpath.
I have this exact same problem with FOP 0.20.1 and Batik. I have my class path setup properly, and FOP does work on other content on that machine. The same exact JARs for FOP and Batik on my Win2000 machine work without any problems. I am using JDK 1.3.1 on Solaris 2.6. I then downloaded and installed the latest Fop with its own version of Batik and I still get the same error indicating org/apache/batik/dom/svg/SVGDOMImplementation is not defined. Using JAR tools I can see and extract that very same class file with out any difficulty. So I believe there really is a bug here.
I had the same problem and installing Xvfb solved it... Try this or PJA or Sun's JDK 1.4 headless option...
This is a setup problem, not a bug in FOP. The documentation explains that a display is required and what jars are needed to run FOP.
batch transition to closed remaining pre-FOP1.0 resolved bugs