I tested with code you committed 3 days ago (in project human-task) in this branch https://github.com/droolsjbpm/jbpm.
I used new classes such as LocalTaskService, SyncWSHumanTaskHandler but I still have the same exception (cf. 1st message) when calling client.complete(task.getId(), "john", null); :(
In this document Not authorized to view the specified article 17148 you can find the simple main I used in my test.
Is there any chance to fix this issue in short term?
Thanks a lot for you help