FXML LoadException when running via JNLP

The attached example, which uses FXML and CSS, works perfectly when launched directly from Eclipse and it also runs perfectly when launched via double-click on the jar file created by the packager (via Tom Schindls Efxclipse plugin) but it fails with a ClassNotFoundException if it is lauched via the generated HTML or JNLP file. Somehow the loader seems to have a problem finding the provided controller class then.