After an update of the Eclipse-BPMN Plugin or the Imixs-BPMN Plugin in some cases it can happen, that an existing process model is no longer loaded correctly into the BPMN editor. In this case it is recommanded to remove the file
org.eclipse.bpmn2.modeler.core.prefs
which is located in the Eclipse workspace under the folder .settings/
After restarting Eclipse the BPMN Plugin is working again correctly.