Estou usando o GAE Python 2.7 com o servidor de desenvolvimento local. Eu configurei um backend

backends:
- name: worker
  class: B1
  options: dynamic

e estou usando a fila de tarefas padrão. Tudo funciona bem e o backend e o taskqueue são visíveis no console do SDK. Além disso, o trabalho de desenvolvimento local é iniciado sem erros:

Multiprocess Setup Complete:
Remote API Server [http://localhost:9200]
App Instance [http://localhost:9000]
Backend Instance: worker.0 [http://localhost:9100]
Backend Balancer: worker [http://localhost:9199]

MAS se eu tentar resolver o backend através de uma tarefa

taskqueue.add(url='/xyz', method='POST', target='worker', params={'a':'b'})

este erro aumenta:

ERROR An error occured while sending the task "task1" (Url: "/backend/languages/create_database/") in queue "default". Treating as a task error.
Traceback (most recent call last):
File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/api/taskqueue/taskqueue_stub.py", line 1884, in ExecuteTask
    connection.endheaders()
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/httplib.py", line 937, in endheaders
    self._send_output(message_body)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/httplib.py", line 797, in _send_output
    self.send(msg)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/httplib.py", line 759, in send
    self.connect()
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/httplib.py", line 740, in connect
    self.timeout, self.source_address)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/socket.py", line 553, in create_connection
    for res in getaddrinfo(host, port, 0, SOCK_STREAM):
gaierror: [Errno 8] nodename nor servname provided, or not known

Eu estou usando 'localhost' e não consigo ver qualquer razão pela qual ele falha. Algumas idéias / soluções? Algum parâmetro de inicialização faltando ou algo parecido?

obrigado