Simulator error UnparseableJsonResponse (“Cannot f

I have successfully tested the first few intents of my app with my webhook in the DialogFlow console, but testing in the Simulator gives the following error:

UnparseableJsonResponse API Version 2: Failed to parse JSON response string with 'INVALID_ARGUMENT' error: ": Cannot find field.".

NB!!! The first thing to notice is that it refers to "API Version 2".

No requests are reaching my webhook - so it appears that this is all within Google.

Using Chrome Developer Tools, I see the Network entry that gets this error response - some details below:

Request url from Simulator: https://assistant.clients6.google.com/v1/assistant:converse?alt=json&key=A.....

NB!!! (Note it says 'v1')

Request Payload:

{"conversationToken":"","debugLevel":1,"inputType":"KEYBOARD","locale":"en-US","mockLocation":{"city":"Mountain View","coordinates":{"latitude":37.421980615353675,"longitude":-122.08419799804688},"formattedAddress":"Googleplex, Mountain View, CA 94043, United States","zipCode":"94043"},"query":"Talk to ","surface":"PHONE"}

Response:

{
    "response": "Connect the docs isn't responding right now. Try again soon.",
    "conversationToken": "GidzaW11bGF0b3JfZGV2aWNlXzM3MTQxRERFM0I0Nzk1Q0ZfMDAwMDA=",
    "audioResponse": "//NExAAR... encoded audio ...",
    "debugInfo": {
        "assistantToAgentDebug": {
            "curlCommand": "curl -v 'https://api.api.ai/api/integrations/google?token=...token...' -H 'Content-Type: application/json;charset=UTF-8' -H 'Google-Actions-API-Version: 2' -H 'Authorization: ...authorization key...' -A 'Mozilla/5.0 (compatible; Google-Cloud-Functions/2.1; +http://www.google.com/bot.html)' -X POST -d '{\"user\":{\"userId\":\"...user id...\",\"locale\":\"en-US\",\"lastSeen\":\"2017-12-15T17:22:55Z\"},\"conversation\":{\"conversationId\":\"1513778713541\",\"type\":\"NEW\"},\"inputs\":[{\"intent\":\"actions.intent.MAIN\",\"rawInputs\":[{\"inputType\":\"KEYBOARD\",\"query\":\"Talk to Connect The Docs\"}]}],\"surface\":{\"capabilities\":[{\"name\":\"actions.capability.AUDIO_OUTPUT\"},{\"name\":\"actions.capability.SCREEN_OUTPUT\"}]},\"isInSandbox\":true,\"availableSurfaces\":[{\"capabilities\":[{\"name\":\"actions.capability.AUDIO_OUTPUT\"},{\"name\":\"actions.capability.SCREEN_OUTPUT\"}]}]}'",
            "assistantToAgentJson": "{\"user\":{\"userId\":\"...user id...\",\"locale\":\"en-US\",\"lastSeen\":\"2017-12-15T17:22:55Z\"},\"conversation\":{\"conversationId\":\"1513778713541\",\"type\":\"NEW\"},\"inputs\":[{\"intent\":\"actions.intent.MAIN\",\"rawInputs\":[{\"inputType\":\"KEYBOARD\",\"query\":\"Talk to Connect The Docs\"}]}],\"surface\":{\"capabilities\":[{\"name\":\"actions.capability.AUDIO_OUTPUT\"},{\"name\":\"actions.capability.SCREEN_OUTPUT\"}]},\"isInSandbox\":true,\"availableSurfaces\":[{\"capabilities\":[{\"name\":\"actions.capability.AUDIO_OUTPUT\"},{\"name\":\"actions.capability.SCREEN_OUTPUT\"}]}]}"
        },
        "agentToAssistantDebug": {
            "agentToAssistantJson": "{\"message\":\"Unexpected apiai response format: Empty speech response\",\"apiResponse\":{\"id\":\"24ddbf1c-3930-40c6-ba50-03c0935cd1d0\",\"timestamp\":\"2017-12-20T14:05:13.766Z\",\"lang\":\"en-us\",\"result\":{},\"status\":{\"code\":200,\"errorType\":\"success\"},\"sessionId\":\"1513778713541\"}}"
        },
        "sharedDebugInfo": [{
            "name": "ResponseValidation",
            "subDebugEntry": [{
                "name": "UnparseableJsonResponse",
                "debugInfo": "API Version 2: Failed to parse JSON response string with 'INVALID_ARGUMENT' error: \": Cannot find field.\"."
            }]
        }]
    },
    "visualResponse": {}
}

I have been informed by Google Support that I am indeed using version V2 - I initiated this in December 2017 - long after the May 2017 "cutoff date" where V2 is supposed to be the default.

Is this a Google bug? Have I missed something setting up my intents? Or is there another setting that may be causing this?

I see that other posts in the DialogFlow forum show the same problem.

Any help is appreciated.

Added on 1/9/2018: Contents of the Debug Tab:

{
  "agentToAssistantDebug": {
    "agentToAssistantJson": {
      "message": "Unexpected apiai response format: Empty speech response",
      "apiResponse": {
        "id": "64a900d2-23e8-4833-b9de-0b207f63bffc",
        "timestamp": "2018-01-08T21:08:36.821Z",
        "lang": "en-us",
        "result": {},
        "status": {
          "code": 200,
          "errorType": "success"
        },
        "sessionId": "1515445716570"
      }
    }
  },
  "assistantToAgentDebug": {
    "assistantToAgentJson": {
      "user": {
        "userId": "ABwppHFGoTJm5fKpau6WWwufKQE5UwkebooowZF7YhvD7PPY-hUfxU2_KRpB0LLNcLPyXasbXnRxXT6fniKk",
        "locale": "en-US",
        "lastSeen": "2018-01-05T15:53:11Z"
      },
      "conversation": {
        "conversationId": "1515445716570",
        "type": "NEW"
      },
      "inputs": [
        {
          "intent": "actions.intent.MAIN",
          "rawInputs": [
            {
              "inputType": "VOICE",
              "query": "talk to connect the docs"
            }
          ]
        }
      ],
      "surface": {
        "capabilities": [
          {
            "name": "actions.capability.AUDIO_OUTPUT"
          },
          {
            "name": "actions.capability.SCREEN_OUTPUT"
          },
          {
            "name": "actions.capability.WEB_BROWSER"
          }
        ]
      },
      "isInSandbox": true,
      "availableSurfaces": [
        {
          "capabilities": [
            {
              "name": "actions.capability.AUDIO_OUTPUT"
            },
            {
              "name": "actions.capability.SCREEN_OUTPUT"
            }
          ]
        }
      ]
    },
    "curlCommand": "curl -v 'https://api.api.ai/api/integrations/google?token=0ffc8bcf72704850a4b4139d49a8d72e' -H 'Content-Type: application/json;charset=UTF-8' -H 'Google-Actions-API-Version: 2' -H 'Authorization: eyJhbGciOiJSUzI1NiIsImtpZCI6IjBhYTQ1NDFlNGM4ZWVhODQ0NjhmZTYxYTkzZmIxYzA2MzJkYjVhMGYifQ.eyJhdWQiOiJhY3RpdmUtZG9jdW1lbnQiLCJpc3MiOiJodHRwczovL2FjY291bnRzLmdvb2dsZS5jb20iLCJqdGkiOiIwY2U2OTdlNmE3NGFiZmVmZTdiYzhmMGU2ZGJlMzEyMDFjOWU3MzA5IiwiaWF0IjoxNTE1NDQ1NzE2LCJleHAiOjE1MTU0NDU4MzYsIm5iZiI6MTUxNTQ0NTQxNn0.hZNpVrH4o8ObGIvZ7BQV44nymekTWR_K4_jsDKCzgj74z57IDyUXNGEZs6KUFxBM_2FXiSoOxJUQZ1OhDRpkpQ6L4LELYN_JDhly7kgy-SLgKgLG6FZ4YV-8qOgr9Uxmr9SsG6NSXdiG7HvTrHLXIwA8K2siBNGGDWAIB691gAC8qsjsq4d3VnHMTeqlJ6mDoOtZ2xdLnJbK5B-OK-rLHEhX6K1-Z7rXQL3OgSwUtRVvYfHI3jqY83Xn3-uf06izkQhwVqH-W6X1REltrlxFTPW2h72D-st-QQ9euIpK3fn0x-z3ouQ17g-rGrPjKcOop9FejtKMT1tibxSkQ7qywQ'  -A 'Mozilla/5.0 (compatible; Google-Cloud-Functions/2.1; +http://www.google.com/bot.html)' -X POST -d '{\"user\":{\"userId\":\"ABwppHFGoTJm5fKpau6WWwufKQE5UwkebooowZF7YhvD7PPY-hUfxU2_KRpB0LLNcLPyXasbXnRxXT6fniKk\",\"locale\":\"en-US\",\"lastSeen\":\"2018-01-05T15:53:11Z\"},\"conversation\":{\"conversationId\":\"1515445716570\",\"type\":\"NEW\"},\"inputs\":[{\"intent\":\"actions.intent.MAIN\",\"rawInputs\":[{\"inputType\":\"VOICE\",\"query\":\"talk to connect the docs\"}]}],\"surface\":{\"capabilities\":[{\"name\":\"actions.capability.AUDIO_OUTPUT\"},{\"name\":\"actions.capability.SCREEN_OUTPUT\"},{\"name\":\"actions.capability.WEB_BROWSER\"}]},\"isInSandbox\":true,\"availableSurfaces\":[{\"capabilities\":[{\"name\":\"actions.capability.AUDIO_OUTPUT\"},{\"name\":\"actions.capability.SCREEN_OUTPUT\"}]}]}'"
  },
  "sharedDebugInfo": [
    {
      "name": "ResponseValidation",
      "subDebugEntry": [
        {
          "debugInfo": "API Version 2: Failed to parse JSON response string with 'INVALID_ARGUMENT' error: \": Cannot find field.\".",
          "name": "UnparseableJsonResponse"
        }
      ]
    }
  ]
}

Contents of the Validation Errors Tab: UnparseableJsonResponse API Version 2: Failed to parse JSON response string with 'INVALID_ARGUMENT' error: ": Cannot find field.".

Screenshot of welcome intent added 1/10/2018:

The problem is a combination of two things:

There are no text replies set in the Response section.
When the Intent is triggered, it does not get sent to a webhook.

As a result, Dialogflow replies to the Assistant with no text response, which is an error.

You can correct this by making sure your welcome intent does one of the following (you don't have to do both):

Set one or more text replies. These would be sent back when the Intent is called.
Check the Use webhook box under Fulfillment. This would call your webhook when the Intent is triggered. (And then make sure that your webhook returns a valid response.)

As you speculated in your comments, you could also change the Welcome Intent to one of your other Intents that you've already tested to respond. There is nothing special about this particular Welcome Intent - it was just the one created by default for you.