Django + Vagrant VM suddenly says Errno 111: connection refused

I'm sure this is my fault, but my system has been working for months and now does not work. I've looked in Django docs and here on stackoverflow, and I understand the issue that 'connection refused' means nobody is listening on the socket; but I don't understand why, and don't know how to go forward.

I use Vagrant + VirtualBox on a MacPro, and for six months I have been able to create virtual machines and run my app with no problems. I have some VM's where everything works as expected.

On my newly-created Vagrant VM's, I install and configure my django apps and run them using 'python manage.py runserver --settings=mumble.settings.py 0.0.0.0:8000'. Note that this is Django 1.8.3, but again,it works fine on some VM's.

For mysterious reasons, when I create a new vagrant VM and install my django payload (using deploy scripts that haven't changed), and connect to the django server, I get Errno 111 Connection refused:

+ python ./manage.py runserver --settings=mydjango.settings 0.0.0.0:8000
              .....
          _d^^^^^^^^^b_
       .d''           ``b.
     .p'                `q.
    .d'                   `b.
   .d'                     `b.   * Mezzanine 4.0.1
   ::                       ::   * Django 1.8.3
  ::    M E Z Z A N I N E    ::  * Python 2.7.12
   ::                       ::   * MySQL 5.7.18
   `p.                     .q'   * Linux 4.4.0-75-generic
    `p.                   .q'
     `b.                 .d'
       `q..          ..p'
          ^q........p^
              ''''

Performing system checks...

System check identified no issues (0 silenced).
May 31, 2017 - 09:50:50
Django version 1.8.3, using settings 'mydjango.settings'
Starting development server at http://0.0.0.0:8000/
Quit the server with CONTROL-C.
Traceback (most recent call last):
  File "/usr/lib/python2.7/wsgiref/handlers.py", line 85, in run
    self.result = application(self.environ, self.start_response)
  File "/vagrant/repos_here/www.mydjango.org/venv/www/local/lib/python2.7/site-packages/django/core/handlers/wsgi.py", line 189, in __call__
    response = self.get_response(request)
  File "/vagrant/repos_here/www.mydjango.org/venv/www/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 204, in get_response
    'request': request
  File "/usr/lib/python2.7/logging/__init__.py", line 1185, in error
    Log 'msg % args' with severity 'ERROR'.
  File "/usr/lib/python2.7/logging/__init__.py", line 1193, in error
    self._log(ERROR, msg, args, **kwargs)
  File "/usr/lib/python2.7/logging/__init__.py", line 1286, in _log
    self.handle(record)
  File "/usr/lib/python2.7/logging/__init__.py", line 1296, in handle
    self.callHandlers(record)
  File "/usr/lib/python2.7/logging/__init__.py", line 1336, in callHandlers
    hdlr.handle(record)
  File "/usr/lib/python2.7/logging/__init__.py", line 759, in handle
    self.emit(record)
  File "/vagrant/repos_here/www.mydjango.org/venv/www/local/lib/python2.7/site-packages/django/utils/log.py", line 129, in emit
    self.send_mail(subject, message, fail_silently=True, html_message=html_message)
  File "/vagrant/repos_here/www.mydjango.org/venv/www/local/lib/python2.7/site-packages/django/utils/log.py", line 132, in send_mail
    mail.mail_admins(subject, message, *args, connection=self.connection(), **kwargs)
  File "/vagrant/repos_here/www.mydjango.org/venv/www/local/lib/python2.7/site-packages/django/core/mail/__init__.py", line 98, in mail_admins
    mail.send(fail_silently=fail_silently)
  File "/vagrant/repos_here/www.mydjango.org/venv/www/local/lib/python2.7/site-packages/django/core/mail/message.py", line 303, in send
    return self.get_connection(fail_silently).send_messages([self])
  File "/vagrant/repos_here/www.mydjango.org/venv/www/local/lib/python2.7/site-packages/django/core/mail/backends/smtp.py", line 100, in send_messages
    new_conn_created = self.open()
  File "/vagrant/repos_here/www.mydjango.org/venv/www/local/lib/python2.7/site-packages/django/core/mail/backends/smtp.py", line 58, in open
    self.connection = connection_class(self.host, self.port, **connection_params)
  File "/usr/lib/python2.7/smtplib.py", line 256, in __init__
    (code, msg) = self.connect(host, port)
  File "/usr/lib/python2.7/smtplib.py", line 316, in connect
    self.sock = self._get_socket(host, port, self.timeout)
  File "/usr/lib/python2.7/smtplib.py", line 291, in _get_socket
    return socket.create_connection((host, port), timeout)
  File "/usr/lib/python2.7/socket.py", line 575, in create_connection
    raise err
error: [Errno 111] Connection refused
[31/May/2017 09:50:55]"GET /donate/ HTTP/1.1" 500 59

Things I know:

'python ./manage.py' with no arguments will run fine, and shows the help message.
All forms of addressing the web server will fail:
- curl http://localhost:8000
- curl http://127.0.0.1:8000
- curl http://192.168.33.10:8000
- from a browser: http://192.168.33.10:8000
nginx is serving on port 80, no problem
my database is a copy of the production database, and it looks fine through 'mysql'

And here is a weird thing: I brought down the Django tutorial, used the 'runserver' command, and it worked!

[vagrant][~/tmp/django_tutorial/mysite]
$ python manage.py runserver 0.0.0.0:8000
Performing system checks...

System check identified no issues (0 silenced).
May 30, 2017 - 20:36:44
Django version 1.8.3, using settings 'mysite.settings'
Starting development server at http://0.0.0.0:8000/
Quit the server with CONTROL-C.
[30/May/2017 20:37:05]"GET / HTTP/1.1" 200 1767

Any suggestions on how to debug this?

Solution

The answer has nothing to do with my environment. My co-worker added a name to settings.ADMINS, which had been an empty list before that. When ADMINS is a non-empty list, any error triggers an attempt to send email to all of the email addresses in ADMINS, and my VM was not configured to send email. This is documented but not emphasized. The problem did not happen to my coworker, because that associated changes did not trigger a server error.