Without further information, my guess is: If you are only using Syncrhonous Tasks, like the WorkItemHandler you shown us, then the process runtime information (including its variables) are never persisted. JBPM5 only stores the information needed to continue with the execution of a process. In your case (again, I'm guessing here since I don't know your complete process) the process runs in one shot: when the execution returns from ksession.startProcess("de.toni.new", params); the process is already finished.
Just for testing purposes try to configure an asynchronous WorkItemHandler to one of your Tasks to see if serialization happens.
Best Regards,