Logs from one of the failed runs:
https://pantheon.corp.google.com/storage/browser/chromeos-autotest-results/13321-chromeos-test

job_reporter_output.log shows that the test run returned a non-0 status.
debug/autoserv.DEBUG shows no problems.
ssp is the culprit:

03/08 05:23:44.504 DEBUG|            zygote:0109| Destroying Zygote container.EFPuTq
03/08 05:23:44.504 DEBUG|         container:0390| Destroying container /usr/local/autotest/containers/container.EFPuTq
03/08 05:23:44.504 DEBUG|             utils:0214| Running 'sudo lxc-destroy -P /usr/local/autotest/containers -n container.EFPuTq -f'
03/08 05:23:45.555 DEBUG|             utils:0214| Running 'sudo -n chown -R 99158 "/usr/local/autotest/results/13321-chromeos-test/chromeos2-row1-rack2-host9"'
03/08 05:23:45.903 DEBUG|             utils:0214| Running 'sudo -n chgrp -R 5000 "/usr/local/autotest/results/13321-chromeos-test/chromeos2-row1-rack2-host9"'
03/08 05:23:46.207 ERROR|         traceback:0013| Traceback (most recent call last):
03/08 05:23:46.207 ERROR|         traceback:0013|   File "/usr/local/autotest/server/autoserv", line 507, in run_autoserv
03/08 05:23:46.208 ERROR|         traceback:0013|     machines)
03/08 05:23:46.208 ERROR|         traceback:0013|   File "/usr/local/autotest/server/autoserv", line 233, in _run_with_ssp
03/08 05:23:46.208 ERROR|         traceback:0013|     test_container.destroy()
03/08 05:23:46.209 ERROR|         traceback:0013|   File "/usr/local/autotest/site_utils/lxc/zygote.py", line 111, in destroy
03/08 05:23:46.209 ERROR|         traceback:0013|     self._cleanup_host_mount()
03/08 05:23:46.209 ERROR|         traceback:0013|   File "/usr/local/autotest/site_utils/lxc/zygote.py", line 221, in _cleanup_host_mount
03/08 05:23:46.209 ERROR|         traceback:0013|     mount.cleanup()
03/08 05:23:46.210 ERROR|         traceback:0013|   File "/usr/local/autotest/site_utils/lxc/utils.py", line 185, in cleanup
03/08 05:23:46.210 ERROR|         traceback:0013|     "cd %s" % self.spec,

Oh, the actual error:
03/08 05:23:46.210 ERROR|         traceback:0013| TypeError: not all arguments converted during string formatting

Reverted blamed CL: https://chromium-review.googlesource.com/c/chromiumos/third_party/autotest/+/955938

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/third_party/autotest/+/ebd19060fcfa21c983fba9d2a39e88a2819aebc5

commit ebd19060fcfa21c983fba9d2a39e88a2819aebc5
Author: Prathmesh Prabhu <pprabhu@chromium.org>
Date: Thu Mar 08 17:31:54 2018

Revert "lxc: consolidate and factor out sudos in utils"

This reverts commit 0ddf0918cc925826ad3aab4bd1dec06621a02615.

Reason for revert: Broke staging.

Original change's description:
> lxc: consolidate and factor out sudos in utils
> 
> Create a helper function to combine commands into a single `sudo bash -c` call.
> This change is relanding a reverted attempt, with less moving pieces.
> 
> BUG=chromium:809306
> BUG= chromium:817569 
> TEST=tryjob
> 
> Change-Id: I0b9d36bbe3732bfb0ca2291ecd6f1e64c6438cee
> Reviewed-on: https://chromium-review.googlesource.com/947192
> Commit-Ready: Jacob Kopczynski <jkop@chromium.org>
> Tested-by: Jacob Kopczynski <jkop@chromium.org>
> Reviewed-by: Ilja H. Friedel <ihf@chromium.org>
> Reviewed-by: Dan Shi <dshi@google.com>

BUG=chromium:809306,  chromium:817569 
BUG= chromium:820112 

Change-Id: I671e64d6e38d68305d997298e320543533800fab
Reviewed-on: https://chromium-review.googlesource.com/955938
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>

[modify] https://crrev.com/ebd19060fcfa21c983fba9d2a39e88a2819aebc5/site_utils/lxc/utils.py