Event Store with exponentially/extreme library execute error

I’m using exponentially/extreme. I just followed the sample codes in the documentation and run eventstore/evenstore as container. This code runs fine:
iex> {:ok, server} = Application.get_env(:extreme, :event_store) |> Extreme.start_link
but I’m getting an error {:error, :not_authenticated} when running
iex> Extreme.execute(server, MyApp.Producer.write_events)

In config.exs

config :extreme, :event_store,
  db_type: :node,
  host: "localhost",
  port: 1113,
  username: "admin",
  password: "changeit",
  reconnect_delay: 2_000,
  connection_name: :my_app,
  max_attempts: :infinity

Is there any config I missed and also what is the username and password referring to? Any sample application using the extreme library would be appreciated. The docu is hard to understand, no typespecs whatsoever. Thanks.