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:
[{"id":-1,"name":"My open issues","jql":"assignee = currentUser() AND resolution = Unresolved order by updated DESC","isSystem":true,"sharePermissions":[],"requiresLogin":true},{"id":-2,"name":"Reported by me","jql":"reporter = currentUser() order by created DESC","isSystem":true,"sharePermissions":[],"requiresLogin":true},{"id":-4,"name":"All issues","jql":"order by created DESC","isSystem":true,"sharePermissions":[],"requiresLogin":false},{"id":-5,"name":"Open issues","jql":"resolution = Unresolved order by priority DESC,updated DESC","isSystem":true,"sharePermissions":[],"requiresLogin":false},{"id":-9,"name":"Done issues","jql":"statusCategory = Done order by updated DESC","isSystem":true,"sharePermissions":[],"requiresLogin":false},{"id":-3,"name":"Viewed recently","jql":"issuekey in issueHistory() order by lastViewed DESC","isSystem":true,"sharePermissions":[],"requiresLogin":false},{"id":-6,"name":"Created recently","jql":"created >= -1w order by created DESC","isSystem":true,"sharePermissions":[],"requiresLogin":false},{"id":-7,"name":"Resolved recently","jql":"resolutiondate >= -1w order by updated DESC","isSystem":true,"sharePermissions":[],"requiresLogin":false},{"id":-8,"name":"Updated recently","jql":"updated >= -1w order by updated DESC","isSystem":true,"sharePermissions":[],"requiresLogin":false}]