MQTT fails on Hassio when upgrading from 0.57.2 to 0.57.3

My configuration file says:

mqtt:
  broker: 127.0.0.1

My MQTT configuration is:

{
  "plain": true,
  "ssl": false,
  "anonymous": true,
  "logins": [],
  "customize": {
    "active": false,
    "folder": "mosquitto"
  },
  "certfile": "fullchain.pem",
  "keyfile": "privkey.pem"
}

Yet MQTT says it’s in the stopped state and my log says:

2017-11-23 11:45:56 ERROR (MainThread) [homeassistant.setup] Error during setup of component mqtt
Traceback (most recent call last):
File “/usr/lib/python3.6/site-packages/homeassistant/setup.py”, line 191, in _async_setup_component
result = yield from component.async_setup(hass, processed_config)
File “/usr/lib/python3.6/site-packages/homeassistant/components/mqtt/init.py”, line 387, in async_setup
success = yield from hass.data[DATA_MQTT].async_connect()
File “/usr/lib/python3.6/site-packages/homeassistant/components/mqtt/init.py”, line 497, in async_connect
self._mqttc.connect, self.broker, self.port, self.keepalive)
File “/usr/lib/python3.6/asyncio/futures.py”, line 331, in iter
yield self # This tells Task to wait for completion.
File “/usr/lib/python3.6/asyncio/tasks.py”, line 244, in _wakeup
future.result()
File “/usr/lib/python3.6/asyncio/futures.py”, line 244, in result
raise self._exception
File “/usr/lib/python3.6/concurrent/futures/thread.py”, line 55, in run
result = self.fn(*self.args, **self.kwargs)
File “/usr/lib/python3.6/site-packages/paho/mqtt/client.py”, line 768, in connect
return self.reconnect()
File “/usr/lib/python3.6/site-packages/paho/mqtt/client.py”, line 895, in reconnect
sock = socket.create_connection((self._host, self._port), source_address=(self._bind_address, 0))
File “/usr/lib/python3.6/socket.py”, line 722, in create_connection
raise err
File “/usr/lib/python3.6/socket.py”, line 713, in create_connection
sock.connect(sa)

Any suggestions? Everything worked fine is 0.57.2.