You'll need to review the QDS Root Log for the date of the issue. By default, the Root Logs are stored in %ProgramData%\QlikTech\DistributionService\1\Log (if a single QDS node). The naming convention for the Root Log is "Root_<date>.txt". Within the log, find the name of the task and see what entries are present. If you can, post the Root Log to this thread.