Thanks Mare and Marco. Actually my code looked like that before, but when I tried this and that as a workaround I finally left (to my shame) the code passing null for the knowledgeBase.
Marco commented on https://issues.jboss.org/browse/JBPM-3383 and found the real problem I ran into originally, which is also a NullPointerException but caused by ConcurrentNodeMemories.getNodeMemory(NodeMemory) line: 65.
Thank you both and sorry for causing additional trouble ;)