[2.2, r5774] Machines don't transition to deploy (but they turn on sometimes?)

I tried to deploy a machine and it didn't transition to 'Deploying'.
Then I tried to commission a machine and it transitioned to 'Commissioning'.
Then I selected 2 machines and tried to deploy again, they never transitioned to 'Deploying' but I saw the following error on the logs:

  4852 Traceback (most recent call last):
  4853 File "/usr/lib/python3/dist-packages/twisted/web/http.py", line 1673, in _finishRequestBody
  4854 self.allContentReceived()
  4855 File "/usr/lib/python3/dist-packages/twisted/web/http.py", line 1736, in allContentReceived
  4856 req.requestReceived(command, path, version)
  4857 File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 86, in requestReceived
  4858 command, path, version)
  4859 File "/usr/lib/python3/dist-packages/twisted/web/http.py", line 762, in requestReceived
  4860 self.process()
  4861 --- <exception caught here> ---
  4862 File "/usr/lib/python3/dist-packages/twisted/web/server.py", line 183, in process
  4863 self.render(resrc)
  4864 File "/usr/lib/python3/dist-packages/twisted/web/server.py", line 234, in render
  4865 body = resrc.render(self)
  4866 File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 115, in wrap_render
  4867 return orig_render(request)
  4868 File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 115, in wrap_render
  4869 return orig_render(request)
  4870 File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 115, in wrap_render

[...]
  5369 return orig_render(request)
  5370 File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 115, in wrap_render
  5371 return orig_render(request)
  5372 File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 115, in wrap_render
  5373 return orig_render(request)
  5374 File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 115, in wrap_render
  5375 return orig_render(request)
  5376 File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 115, in wrap_render
  5377 return orig_render(request)
  5378 File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 115, in wrap_render
  5379 return orig_render(request)
  5380 File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 115, in wrap_render
  5381 return orig_render(request)
  5382 File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 115, in wrap_render
  5383 return orig_render(request)
  5384 File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 115, in wrap_render
  5385 return orig_render(request)
  5386 File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 115, in wrap_render
  5387 return orig_render(request)
  5388 File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 115, in 5482 File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 115, in wrap_render
[..]
  5483 return orig_render(request)
  5484 File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 115, in wrap_render
  5485 return orig_render(request)
  5486 File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 115, in wrap_render
  5487 return orig_render(request)
  5488 File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 115, in wrap_render
  5489 return orig_render(request)
  5490 File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 115, in wrap_render
  5491 return orig_render(request)
  5492 File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 115, in wrap_render
  5493 return orig_render(request)
  5494 File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 115, in wrap_render
  5495 return orig_render(request)
  5496 File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 115, in wrap_render
  5497 return orig_render(request)
  5498 File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 115, in wrap_render
  5499 return orig_render(request)
  5500 File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 115, in wrap_render
  5501 return orig_render(request)
  5502 File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 115, in wrap_render
  5503 return orig_render(request)
  5504 File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 115, in wrap_render
  5505 return orig_render(request)
  5506 File "/usr/lib/python3/dist-packages/maasserver/webapp.py", line 115, in wrap_render
  5507 return orig_render(request)
  5508 File "/usr/lib/python3/dist-packages/twisted/web/resource.py", line 250, in render
  5509 return m(request)
  5510 File "/usr/lib/python3/dist-packages/metadataserver/api_twisted.py", line 140, in render_POST
  5511 message = json.loads(payload)
  5512 File "/usr/lib/python3.5/json/__init__.py", line 319, in loads
  5513 return _default_decoder.decode(s)
  5514 File "/usr/lib/python3.5/json/decoder.py", line 339, in decode
  5515 obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  5516 File "/usr/lib/python3.5/json/decoder.py", line 355, in raw_decode
  5517 obj, end = self.scan_once(s, idx)
  5518 builtins.RecursionError: maximum recursion depth exceeded while decoding a JSON object from a unicode string
  5519
  5520 2017-03-02 11:00:49 twisted.python.log: [info] ::ffff:10.90.90.192 - - [02/Mar/2017:16:00:49 +0000] "POST /MAAS/metadata/status/xw8mcp HTTP/1.1" 500 272406 "-" "python-requests/2.9.1"