GAE / Go：/ _ ah / start返回401

Our GAE/Go application returns 401 response to /_ah/start like below.

app.yaml is this.

runtime: go
api_version: go1
application: (out app)
version: 777
threadsafe: true
manual_scaling:
  instances: 1  

handlers:

- url: /_ah/.*
  script: _go_app

- url: /.*
  script: _go_app
  login: required
  secure: always

Why we have this error? How can I fix this? We know deleting manual_scaling can fix this error, but we would like to restrict the number of our instance.