Those errors are most of hte time because you don't have the correct users in the database, and for that reason a FK is violated.
Make sure that you have inserted and commited to the database the Administrator user and the user that you are assigning into the task.
Without a test (inside a maven project) which demosntrate the error is almost impossible to help you to solve a problem, which looks like a configuration/start up problem in the user side, not in jBPM side.
Cheers