The bot for chromeos6-row3-rack12-host13 https://chrome-swarming.appspot.com/bot?id=chromeos-skylab-bot-cbece344-04a4-463a-b440-7d105b6cc845 has been quarantined, which is WAI.

Bot events show the same stack trace as OP: 
Failed to call hook get_state(): No JSON object could be decoded
Traceback (most recent call last):
File "/usr/local/google/home/chromeos-test/skylab_bots/d7863/swarming_bot.1.zip/bot_code/bot_main.py", line 293, in _call_hook_safe
return _call_hook(chained, botobj, name, *args)
File "/usr/local/google/home/chromeos-test/skylab_bots/d7863/swarming_bot.1.zip/bot_code/bot_main.py", line 164, in hook
return func(chained, botobj, name, *args, **kwargs)
File "/usr/local/google/home/chromeos-test/skylab_bots/d7863/swarming_bot.1.zip/bot_code/bot_main.py", line 260, in _call_hook
return hook(botobj, *args, **kwargs)
File "injected.py", line 110, in get_state
File "injected.py", line 132, in _get_cached_state
File "injected.py", line 161, in _get_cached_botinfo
File "/usr/lib/python2.7/json/__init__.py", line 290, in load
**kw)
File "/usr/lib/python2.7/json/__init__.py", line 338, in loads
return _default_decoder.decode(s)
File "/usr/lib/python2.7/json/decoder.py", line 366, in decode
obj, end = self.raw_decode(s, idx=_w(s, 0).end())
File "/usr/lib/python2.7/json/decoder.py", line 384, in raw_decode
raise ValueError("No JSON object could be decoded")
ValueError: No JSON object could be decoded
Calling stack:
0 api/bot.py:230:post_error()
1 bot_code/bot_main.py:298:_call_hook_safe()
2 bot_code/bot_main.py:366:_get_state()
3 bot_code/bot_main.py:1066:_run_bot_inner()
4 bot_code/bot_main.py:966:_run_bot()
5 bot_code/bot_main.py:1402:main()
6 __main__.py:166:CMDstart_bot()
7 __main__.py:254:main()
8 __main__.py:266:<module>()
9 runpy.py:72:_run_code()
10 runpy.py:162:_run_module_as_main()

This is because the cached swarming state for the bot is empty.

chromeos-test@pprabhu-skylab-drone-2:/usr/local/autotest/swarming_state$ cat cbece344-04a4-463a-b440-7d105b6cc845.json

The last task to run on the bot was https://chrome-swarming.appspot.com/task?id=3ec98641bce0dd10&refresh=10&show_raw=1

This task was not able to correctly persist the swarming state on the bot due to  issue 865561 

To recover all the quarantined bots, we need to simply stop the bot. It will be restarted by skylab_swarming_manager with fresh cached bot state.

Sadly, the "shut down bot gracefully" functionality simply creates a task that would quit the bot. But that task will not run because the bot is quarantined: https://chrome-swarming.appspot.com/task?id=3ecdb4981645a810&refresh=10&show_raw=1

Another "clean" way to recover these bots is to remove them from the inventory, then re-add them once skylab_swarming_manager shuts down the bots.

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chromeos/infra_internal/skylab_inventory/+/1201ace97fdaa6e4f17059558f6118bc1ba2b622

commit 1201ace97fdaa6e4f17059558f6118bc1ba2b622
Author: Prathmesh Prabhu <pprabhu@chromium.org>
Date: Thu Jul 19 23:37:00 2018

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chromeos/infra_internal/skylab_inventory/+/ebebe4fb41684627faffc270bc8fe54190638be7

commit ebebe4fb41684627faffc270bc8fe54190638be7
Author: Prathmesh Prabhu <pprabhu@google.com>
Date: Thu Jul 19 23:43:14 2018

After #5 and #6, the bots are all back.
So, removing and adding the DUTs back works.