Could it be, that the intermediate throw event has to be registered for "Send Task" ?
//This works
ksession.getWorkItemManager().registerWorkItemHandler("Send Task", new SendTaskHandler());
//This works
ksession.getWorkItemManager().registerWorkItemHandler("Send Task", new MyOwnIntermediateThrowHandler());
//Does not work
ksession.getWorkItemManager().registerWorkItemHandler("Send Task", new MyOwnIntermediateThrowHandler());