Local Parse Server with Flutter causes error 111

Question

I have a local Parse Server installed on Ubuntu 20.04 LTS with:

npm install -g --save --save-exact parse-server@4.2.0

Everything works with it using a locally installed Parse dashboard.

Running a Flutter app, however,produces an error:

DioError [DioErrorType.DEFAULT]: SocketException: OS Error: Connection refused, errno = 111, address = localhost, port = 60050

The local Parse Server ins started with:

mongodb-runner start

parse-server parse-server-config.json --cloud ~/main.js

The config is taken from a JSON file:

{
  "appName": "kayman",
  "databaseURI": "mongodb://localhost:27017/parsedb",
  "appId": "test",
  "javascriptKey": "test",
  "masterKey": "test",
  "serverURL": "http://localhost:1337/parse",
  "publicServerURL": "http://0.0.0.0:1337/parse",
  "port": 1337
}

SDK initialisation:

_parse = await Parse().initialize(
      serverConfig.appId,
      serverConfig.serverUrl,
      autoSendSessionId: true,
      debug: config.debug,
      coreStore: _coreStore,
      clientKey: serverConfig.clientId,
    );

config is from this section of a JSON file:

  "test": {
    "applicationId": "test",
    "clientId": "test",
    "restId": "test",
    "serverUrl": "http://localhost:1337/parse/"
  },

This post seems relevant, but I cannot find a mongodb.conf file anywhere. The only reference to mongo I can find is in ~/.mongodb, but that does not appear to contain a config file.

Can anyone advise how to fix this?