After chatting with Newell, I looked at the apt history log and it shows three recent updates to MAAS:
Start-Date: 2017-08-23  10:54:10
Commandline: apt install maas
Requested-By: ubuntu (1000)
Upgrade: python-curtin:amd64 (0.1.0~bzr511-0ubuntu1~ubuntu16.04.1, 0.1.0~bzr518-0ubuntu1~ubuntu16.04.1), python3-maas-provisioningserver:amd64 (2.3.0~alpha1-6163-gd7f5060-0ubuntu1~16.04.1, 2.3.0~alpha3-6199-g057d34c-0ubuntu1~16.04.1), python3-django-maas:amd64 (2.3.0~alpha1-6163-gd7f5060-0ubuntu1~16.04.1, 2.3.0~alpha3-6199-g057d34c-0ubuntu1~16.04.1), python3-curtin:amd64 (0.1.0~bzr511-0ubuntu1~ubuntu16.04.1, 0.1.0~bzr518-0ubuntu1~ubuntu16.04.1), maas-common:amd64 (2.3.0~alpha1-6163-gd7f5060-0ubuntu1~16.04.1, 2.3.0~alpha3-6199-g057d34c-0ubuntu1~16.04.1), maas-rack-controller:amd64 (2.3.0~alpha1-6163-gd7f5060-0ubuntu1~16.04.1, 2.3.0~alpha3-6199-g057d34c-0ubuntu1~16.04.1), maas-cli:amd64 (2.3.0~alpha1-6163-gd7f5060-0ubuntu1~16.04.1, 2.3.0~alpha3-6199-g057d34c-0ubuntu1~16.04.1), maas-dns:amd64 (2.3.0~alpha1-6163-gd7f5060-0ubuntu1~16.04.1, 2.3.0~alpha3-6199-g057d34c-0ubuntu1~16.04.1), maas:amd64 (2.3.0~alpha1-6163-gd7f5060-0ubuntu1~16.04.1, 2.3.0~alpha3-6199-g057d34c-0ubuntu1~16.04.1), python3-maas-client:amd64 (2.3.0~alpha1-6163-gd7f5060-0ubuntu1~16.04.1, 2.3.0~alpha3-6199-g057d34c-0ubuntu1~16.04.1), maas-region-controller:amd64 (2.3.0~alpha1-6163-gd7f5060-0ubuntu1~16.04.1, 2.3.0~alpha3-6199-g057d34c-0ubuntu1~16.04.1), maas-dhcp:amd64 (2.3.0~alpha1-6163-gd7f5060-0ubuntu1~16.04.1, 2.3.0~alpha3-6199-g057d34c-0ubuntu1~16.04.1), maas-region-api:amd64 (2.3.0~alpha1-6163-gd7f5060-0ubuntu1~16.04.1, 2.3.0~alpha3-6199-g057d34c-0ubuntu1~16.04.1), curtin-common:amd64 (0.1.0~bzr511-0ubuntu1~ubuntu16.04.1, 0.1.0~bzr518-0ubuntu1~ubuntu16.04.1), maas-proxy:amd64 (2.3.0~alpha1-6163-gd7f5060-0ubuntu1~16.04.1, 2.3.0~alpha3-6199-g057d34c-0ubuntu1~16.04.1)
End-Date: 2017-08-23  11:01:14

Start-Date: 2017-08-29  11:08:47
Commandline: apt install maas
Requested-By: ubuntu (1000)
Upgrade: python3-maas-provisioningserver:amd64 (2.3.0~alpha3-6199-g057d34c-0ubuntu1~16.04.1, 2.3.0~alpha3-6214-g6c2ae15-0ubuntu1~16.04.1), python3-django-maas:amd64 (2.3.0~alpha3-6199-g057d34c-0ubuntu1~16.04.1, 2.3.0~alpha3-6214-g6c2ae15-0ubuntu1~16.04.1), maas-common:amd64 (2.3.0~alpha3-6199-g057d34c-0ubuntu1~16.04.1, 2.3.0~alpha3-6214-g6c2ae15-0ubuntu1~16.04.1), maas-rack-controller:amd64 (2.3.0~alpha3-6199-g057d34c-0ubuntu1~16.04.1, 2.3.0~alpha3-6214-g6c2ae15-0ubuntu1~16.04.1), maas-cli:amd64 (2.3.0~alpha3-6199-g057d34c-0ubuntu1~16.04.1, 2.3.0~alpha3-6214-g6c2ae15-0ubuntu1~16.04.1), maas-dns:amd64 (2.3.0~alpha3-6199-g057d34c-0ubuntu1~16.04.1, 2.3.0~alpha3-6214-g6c2ae15-0ubuntu1~16.04.1), maas:amd64 (2.3.0~alpha3-6199-g057d34c-0ubuntu1~16.04.1, 2.3.0~alpha3-6214-g6c2ae15-0ubuntu1~16.04.1), python3-maas-client:amd64 (2.3.0~alpha3-6199-g057d34c-0ubuntu1~16.04.1, 2.3.0~alpha3-6214-g6c2ae15-0ubuntu1~16.04.1), maas-region-controller:amd64 (2.3.0~alpha3-6199-g057d34c-0ubuntu1~16.04.1, 2.3.0~alpha3-6214-g6c2ae15-0ubuntu1~16.04.1), maas-dhcp:amd64 (2.3.0~alpha3-6199-g057d34c-0ubuntu1~16.04.1, 2.3.0~alpha3-6214-g6c2ae15-0ubuntu1~16.04.1), maas-region-api:amd64 (2.3.0~alpha3-6199-g057d34c-0ubuntu1~16.04.1, 2.3.0~alpha3-6214-g6c2ae15-0ubuntu1~16.04.1), maas-proxy:amd64 (2.3.0~alpha3-6199-g057d34c-0ubuntu1~16.04.1, 2.3.0~alpha3-6214-g6c2ae15-0ubuntu1~16.04.1)
End-Date: 2017-08-29  11:13:07

Start-Date: 2017-08-31  08:43:14
Commandline: apt install maas
Requested-By: ubuntu (1000)
Upgrade: python3-maas-provisioningserver:amd64 (2.3.0~alpha3-6214-g6c2ae15-0ubuntu1~16.04.1, 2.3.0~alpha3-6223-gaca9fbb-0ubuntu1~16.04.1), python3-django-maas:amd64 (2.3.0~alpha3-6214-g6c2ae15-0ubuntu1~16.04.1, 2.3.0~alpha3-6223-gaca9fbb-0ubuntu1~16.04.1), maas-common:amd64 (2.3.0~alpha3-6214-g6c2ae15-0ubuntu1~16.04.1, 2.3.0~alpha3-6223-gaca9fbb-0ubuntu1~16.04.1), maas-rack-controller:amd64 (2.3.0~alpha3-6214-g6c2ae15-0ubuntu1~16.04.1, 2.3.0~alpha3-6223-gaca9fbb-0ubuntu1~16.04.1), maas-cli:amd64 (2.3.0~alpha3-6214-g6c2ae15-0ubuntu1~16.04.1, 2.3.0~alpha3-6223-gaca9fbb-0ubuntu1~16.04.1), maas-dns:amd64 (2.3.0~alpha3-6214-g6c2ae15-0ubuntu1~16.04.1, 2.3.0~alpha3-6223-gaca9fbb-0ubuntu1~16.04.1), maas:amd64 (2.3.0~alpha3-6214-g6c2ae15-0ubuntu1~16.04.1, 2.3.0~alpha3-6223-gaca9fbb-0ubuntu1~16.04.1), python3-maas-client:amd64 (2.3.0~alpha3-6214-g6c2ae15-0ubuntu1~16.04.1, 2.3.0~alpha3-6223-gaca9fbb-0ubuntu1~16.04.1), maas-region-controller:amd64 (2.3.0~alpha3-6214-g6c2ae15-0ubuntu1~16.04.1, 2.3.0~alpha3-6223-gaca9fbb-0ubuntu1~16.04.1), maas-dhcp:amd64 (2.3.0~alpha3-6214-g6c2ae15-0ubuntu1~16.04.1, 2.3.0~alpha3-6223-gaca9fbb-0ubuntu1~16.04.1), maas-region-api:amd64 (2.3.0~alpha3-6214-g6c2ae15-0ubuntu1~16.04.1, 2.3.0~alpha3-6223-gaca9fbb-0ubuntu1~16.04.1), maas-proxy:amd64 (2.3.0~alpha3-6214-g6c2ae15-0ubuntu1~16.04.1, 2.3.0~alpha3-6223-gaca9fbb-0ubuntu1~16.04.1)
End-Date: 2017-08-31  08:50:05

I am "pretty sure" this was working earlier on 2.3.0~alpha3-6199-g057d34c-0ubuntu1~16.04.1, so perhaps one of the two more recent ones broke something?

The complaint that triggered this bug came in just last night, so it's possible the break occurred on the update dated 8/29, I did not update this on 8/31, I'm not sure who did either of those, to be honest.