Calling TaskService.complete(...) is failing, and process is not continuing in BPMS 6.1

Issue

We have severals tasks in jBPM that are stuck in "InProgress" status. We have a custom web service that calls the TaskService.complete(...) method to complete the human task, and to proceed with the process execution, but intermittently it fails with the following exception:

WARN  [main] [org.jbpm.services.task.persistence.TaskTransactionInterceptor] Could not commit session
java.lang.NullPointerException: null
    at org.jbpm.services.task.wih.ExternalTaskEventListener.afterTaskCompletedEvent(ExternalTaskEventListener.java:116) ~[jbpm-human-task-workitems-6.2.0.Final-redhat-6.jar:6.2.0.Final-redhat-6]
    at org.jbpm.services.task.events.TaskEventSupport.fireAfterTaskCompleted(TaskEventSupport.java:211) ~[jbpm-human-task-core-6.2.0.Final-redhat-6.jar:6.2.0.Final-redhat-6]
    at org.jbpm.services.task.internals.lifecycle.MVELLifeCycleManager.taskOperation(MVELLifeCycleManager.java:359) ~[jbpm-human-task-core-6.2.0.Final-redhat-6.jar:6.2.0.Final-redhat-6]
    at org.jbpm.services.task.impl.TaskInstanceServiceImpl.complete(TaskInstanceServiceImpl.java:153) ~[jbpm-human-task-core-6.2.0.Final-redhat-6.jar:6.2.0.Final-redhat-6]
    at org.jbpm.services.task.commands.CompleteTaskCommand.execute(CompleteTaskCommand.java:76) ~[jbpm-human-task-core-6.2.0.Final-redhat-6.jar:6.2.0.Final-redhat-6]
    at org.jbpm.services.task.commands.CompleteTaskCommand.execute(CompleteTaskCommand.java:39) ~[jbpm-human-task-core-6.2.0.Final-redhat-6.jar:6.2.0.Final-redhat-6]
    at org.jbpm.services.task.commands.CompositeCommand.execute(CompositeCommand.java:127) ~[jbpm-human-task-core-6.2.0.Final-redhat-6.jar:6.2.0.Final-redhat-6]
    at org.jbpm.services.task.commands.TaskCommandExecutorImpl$SelfExecutionCommandService.execute(TaskCommandExecutorImpl.java:65) ~[jbpm-human-task-core-6.2.0.Final-redhat-6.jar:6.2.0.Final-redhat-6]
    at org.drools.core.command.impl.AbstractInterceptor.executeNext(AbstractInterceptor.java:41) [drools-core-6.2.0.Final-redhat-6.jar:6.2.0.Final-redhat-6]
    at org.jbpm.services.task.persistence.TaskTransactionInterceptor.execute(TaskTransactionInterceptor.java:54) ~[jbpm-human-task-jpa-6.2.0.Final-redhat-6.jar:6.2.0.Final-redhat-6]
    at org.drools.core.command.impl.AbstractInterceptor.executeNext(AbstractInterceptor.java:41) [drools-core-6.2.0.Final-redhat-6.jar:6.2.0.Final-redhat-6]
    at org.drools.persistence.jta.TransactionLockInterceptor.execute(TransactionLockInterceptor.java:67) [drools-persistence-jpa-6.2.0.Final-redhat-6.jar:6.2.0.Final-redhat-6]
    at org.jbpm.services.task.commands.TaskCommandExecutorImpl.execute(TaskCommandExecutorImpl.java:40) [jbpm-human-task-core-6.2.0.Final-redhat-6.jar:6.2.0.Final-redhat-6]
    at org.jbpm.services.task.impl.command.CommandBasedTaskService.complete(CommandBasedTaskService.java:156) [jbpm-human-task-core-6.2.0.Final-redhat-6.jar:6.2.0.Final-redhat-6]
    at com.sample.ProcessJPAContinueTest.testProcess(ProcessJPAContinueTest.java:105) [test-classes/:na]