Error Message :
U00007039 Object '' could not be restarted. Reference object '' (RunID '') is still active.
When trying to restart a run in Post Process with
:SET &RESTART_RC# = RESTART_UC_OBJECT(&$NAME#,LAST,,,&$QUEUE#)
the above error message is displayed.
Using "LAST" restarts the Last execution of the Object, this could be a task that is not restartable when there are multiple parallel starts.
This does not happen with tasks that are spread out or single executions, only with parallel executions.