I just noticed that the problem persists (pun intended) when I remove the timer event from the diagram. Even if the process has only two script services loadStatefulKnowledgeSession fails with the exception above. Looking at the rare examples that I find I can't see any significant differences. The problem occurs only when an instance was or is active, otherwise restoring the knowlede session works (but is pointless of course).
Has anyone a clue how I can make it work?