Saxon

Your extension relies on a very, very old version of Saxon. The configuration you're using adds not just a very old version of Saxon to the classpath, it also adds old versions of ICU4J and Xerces. That's just not going to work reliably.

Your extension won't load without the com.icl.saxon.style.ExtensionElementFactory class and appears to be based off of Saxon 6.5.5. I'm not sure exactly what your extension does, but it appears to attempt to load the stylesheets with some classes of that vintage. That fails, but it's impossible to say if that's because of interactions between the current version of Xerces and the old one, or if that version of Saxon is so old that it's actually using a different parser.

I can't explain in any detail why it works with 10.8 and not 11.4. My guess is "luck". Because you have mutiple implementations of the same classes on the classpath, what happens is going to depend on how the JVM happens to decide to load the jar files and search the classpath.

I recommend that you update the implementation of the extension classes so that they work with the current version of Saxon. Then you won't need to put all those conflicting jar files on the classpath and the results will be more reliable.

Hope that's useful and sorry we can't do more.