Webhooks Failing with HTTP Response - 1

Webhooks Failing with HTTP Response - 1

I am attempting to create a sample test webhook for my application. The webhook fires and reports an HTTP Response - 1.  I'm not able to tell where this is coming from as I have extensively tested this API from multiple IP addresses and it always returns status 200.

The webhook is a GET with one parameter - the Account ID.  I don't want the post the actual URL here, but it looks exatly like this: 
https://www.example.com/api/ping?AccountID=123456  

The Preview looks like this 
https://www.example.com/api/ping?AccountID=${Accounts.Account Id}

When I call that API from a browser or cURL, it returns an http status 200, and a JSON representation of the data sent like so:  {"AccountID":"123456"}

There must be something obvious that I'm missing here.