What causes org.hibernate.StaleObjectStateException while working with jBPM5 ?

Issue

• I have created a business process in jBPM5. After processing the flow for few times the flow is giving the following ERROR.
  Just for your information I am running on WebSphere Application Server (WAS) 8 , and I am using the jBPM 5.x which comes with JBoss Enterprise BRMS Platform 5.3 .
  Can you please advise me what is causing this ERROR?

java.lang.RuntimeException: Could not commit transaction
    at org.jbpm.task.service.TaskServiceSession.doOperationInTransaction(TaskServiceSession.java:1011)
    at org.jbpm.task.service.TaskServiceSession.getContent(TaskServiceSession.java:589)
    at org.jbpm.task.service.local.LocalTaskService.getContent(LocalTaskService.java:130)
    at org.jbpm.process.workitem.wsht.SyncWSHumanTaskHandler$TaskCompletedHandler.handleCompletedTask(SyncWSHumanTaskHandler.java:343)
        .....
Caused by: java.lang.RuntimeException: Unable to commit transaction
    at org.drools.persistence.jta.JtaTransactionManager.commit(JtaTransactionManager.java:182)
    at org.jbpm.task.service.persistence.TaskJTATransactionManager.commit(TaskJTATransactionManager.java:56)
    at org.jbpm.task.service.persistence.TaskPersistenceManager.endTransaction(TaskPersistenceManager.java:94)
    at org.jbpm.task.service.TaskServiceSession.doOperationInTransaction(TaskServiceSession.java:999)
        .....
Caused by: javax.transaction.RollbackException
    at com.ibm.tx.jta.impl.TransactionImpl.stage3CommitProcessing(TransactionImpl.java:1258)
    at com.ibm.tx.jta.impl.TransactionImpl.processCommit(TransactionImpl.java:1028)
    at com.ibm.tx.jta.impl.TransactionImpl.commit(TransactionImpl.java:949)
        .....
Caused by: javax.persistence.OptimisticLockException
    at org.hibernate.ejb.AbstractEntityManagerImpl.wrapStaleStateException(AbstractEntityManagerImpl.java:627)
    at org.hibernate.ejb.AbstractEntityManagerImpl.throwPersistenceException(AbstractEntityManagerImpl.java:588)
    at org.hibernate.ejb.AbstractEntityManagerImpl$1.beforeCompletion(AbstractEntityManagerImpl.java:513)
        .....    
Caused by: org.hibernate.StaleObjectStateException: Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect): [org.jbpm.task.Task#5]
    at org.hibernate.persister.entity.AbstractEntityPersister.check(AbstractEntityPersister.java:1782)
    at org.hibernate.persister.entity.AbstractEntityPersister.update(AbstractEntityPersister.java:2425)
    at org.hibernate.persister.entity.AbstractEntityPersister.updateOrInsert(AbstractEntityPersister.java:2325)
    at org.hibernate.persister.entity.AbstractEntityPersister.update(AbstractEntityPersister.java:2625)
    at org.hibernate.action.EntityUpdateAction.execute(EntityUpdateAction.java:115)
    at org.hibernate.engine.ActionQueue.execute(ActionQueue.java:279)
        .....