I tried this (json both in and out) and I got this response:

        "unsubscribeContextResponse" : {
          "subscriptionId" : "0",
          "statusCode" : {
            "code" : "500",
            "reasonPhrase" : "Parse Error",
            "details" : "JSON parse error: <unspecified file>(1): expected object or array"
          }
        }

Why did we not see this before?
No idea ...