This was uncovered in the context of https://github.com/jenkinsci/kubernetes-plugin/pull/127 - PluginFirstClassLoader.findResource always returns null, which seems to result in CpsScript.getClass().getClassLoader().loadClass("some.class.in.resources.groovy") failing (for that matter, UberClassLoader.findResource("some/file/from/a/PluginFirstClassLoader") always returns null, which I'm guessing is the problem). Needs more investigation to figure out exactly what's happening here.