As described in Dependencies and Class Loading:
Java defines a Thread.getContextClassLoader(). Jenkins does not use this much; it will normally be set by the servlet container to the Jenkins core loader.
Unfortunately, JAXB has a design flaw in that JAXBContext#newInstance expects JAXB to be available in the thread's context class loader. With JAXB being detached to a separate plugin as of recent Java 11 changes, JAXB is no longer available in the Jenkins core loader. This causes a failure to load JAXB.
This plugin has been identified as containing such a call to JAXBContext#newInstance The suggested solution is to wrap calls to JAXBContext#newInstance with a try/finally block to set the thread's context class loader to that of the plugin, which (by virtue of a plugin-to-plugin dependency on JAXB API plugin) enables JAXB to be loaded. See the following PR for an example: