From the failure:
240731895 wolf-paladin/R71-11083.0.0-rc1/bvt-inline/security_SymlinkRestrictions started on: 2018-09-20 07:22:00 status Completed
240731850 wolf-paladin/R71-11083.0.0-rc1/bvt-inline/login_VMSanity started on: 2018-09-20 07:18:22 status Completed
240728868 wolf-paladin/R71-11083.0.0-rc1/provision/dummy_Pass started on: 2018-09-20 07:06:57 status Completed
1982935 Provision started on: 2018-09-20 07:00:11 status PASS
Traceback (most recent call last):
  File "/usr/local/autotest/site_utils/run_suite.py", line 2076, in <module>
    sys.exit(main())
  File "/usr/local/autotest/site_utils/run_suite.py", line 2065, in main
    result = _run_task(options)
  File "/usr/local/autotest/site_utils/run_suite.py", line 2000, in _run_task
    return _run_suite(options)
  File "/usr/local/autotest/site_utils/run_suite.py", line 1773, in _run_suite
    return _handle_job_wait(afe, job_id, options, job_timer, is_real_time)
  File "/usr/local/autotest/site_utils/run_suite.py", line 1906, in _handle_job_wait
    rpc_helper.diagnose_pool(options.dependencies, cutoff)
  File "/usr/local/autotest/site_utils/diagnosis_utils.py", line 251, in diagnose_pool
    for job in history:
  File "/usr/local/autotest/server/lib/status_history.py", line 504, in __iter__
    self.end_time)
  File "/usr/local/autotest/server/lib/status_history.py", line 495, in _get_history
    self._afe, self._host.id, start_time, end_time)
  File "/usr/local/autotest/server/lib/status_history.py", line 357, in get_hqes
    return [cls(hqe) for hqe in hqelist]
  File "/usr/local/autotest/server/lib/status_history.py", line 363, in __init__
    hqe.started_on, hqe.finished_on)
  File "/usr/local/autotest/server/lib/status_history.py", line 134, in __init__
    self.end_time = parse_time(end_time)
  File "/usr/local/autotest/server/lib/status_history.py", line 79, in parse_time
    return int(time_utils.to_epoch_time(time_string))
  File "/usr/local/autotest/client/common_lib/time_utils.py", line 101, in to_epoch_time
    'number. Unexpected value: %s.' % value)
ValueError: Value should be a datetime object, string or a number. Unexpected value: None.
07:39:05: INFO: Re-run swarming_cmd to avoid buildbot salency check.

It looks like a timeout occurred and the exception happened after that.