Thanks for your help so far! I found that the problem occurs when I recreate the KnowledgeBase. If I keep the KnowledgeBase (e.g. in a member variable) I can recreate the session with JPAKnowledgeService.loadStatefulKnowledgeSession and everything is fine.
Here's how I create the KnowledgeBase:
{code}
KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder();
kbuilder.add(ResourceFactory.newClassPathResource("TimerPersistence.bpmn2"), ResourceType.BPMN2);
KnowledgeBase knowledgeBase = kbuilder.newKnowledgeBase();
{code}
How can I handle recreation of the KnowledgeBase e.g. in a web app when I run in a new java process?