ArcGIS Rest API in C#. submitting the job to ConnectOriginsToDestinations service

Question

I'm getting started with the online ArcGIS analysis services API. I've hit a dead-end with the connectOriginsToDestinations analysis task; even with a minimal request, I get a job failure with the response "Operation failed. Invalid URL (code = 400)".

Unfortunately, that's not much to go on, as I can't figure out from the error exactly which field in the job request the api is complaining about. The response from the job submission is "esriJobSubmitted", with a jobId to lookup, but when polling for job status, the first poll always gives me the failure message.

{
      "type": "esriJobMessageTypeError",
      "description": "Operation failed. Invalid URL (code = 400)."
}

Request parameters:

Job submission URL: >>https://analysis2.arcgis.com/arcgis/rest/services/tasks/GPServer/connectOriginsToDestinations/submitJob?

Job status poll URL : >>https://analysis2.arcgis.com/arcgis/rest/services/tasks/GPServer/connectOriginsToDestinations/jobs/jc004ad8813c14d75b4bf157c43b2149d?f=json&token=MYTOKEN

Submitted Task contents (a form-encoded POST body with the following fields):

f: json
token: <my token>
originsLayer: <json blob below>
destinationsLayer: <json blob below>
measurementType: "StraightLine"

Detail: originsLayer:
{
  "layerDefinition": {
    "geometryType": "esriGeometryPoint",
    "fields": [
      {
        "name": "Id",
        "type": "esriFieldTypeString",
        "alias": "Id"
      }
    ]
  },
  "featureSet": {
    "geometryType": "esriGeometryPoint",
    "spatialReference": {
      "wkid": 4326
    },
    "features": [
      {
        "geometry": {
          "x": -77.694145,
          "y": 45.478969
        },
        "attributes": {
          "Id": "id1"
        }
      }
    ]
  }
}

Detail: destinationsLayer:
{
  "layerDefinition": {
    "geometryType": "esriGeometryPoint",
    "fields": [
      {
        "name": "Id",
        "type": "esriFieldTypeString",
        "alias": "Id"
      }
    ]
  },
  "featureSet": {
    "geometryType": "esriGeometryPoint",
    "spatialReference": {
      "wkid": 4326
    },
    "features": [
      {
        "geometry": {
          "x": -77.694145,
          "y": 45.478969
        },
        "attributes": {
          "Id": "id1"
        }
      },
      {
        "geometry": {
          "x": -77.476424,
          "y": 46.090899
        },
        "attributes": {
          "Id": "id2"
        }
      }
    ]
  }
}