Outlook Notify v2 API - Create calendar subscription - outlook-restapi

Question

I am setting up a subscription request. I receive the validationtoken url param so according to the docs I have to reply in the next five seconds doing this:
Set the content type in the response header to text\plain.
Include only the validation token in the response body.
Return an HTTP 200 response code.
This tells the Outlook notifications service that the listener received the URL validation request, and the validation succeeded.
I am receiving an error though:
{"error":{"code":"ErrorInvalidParameter","message":"Notification URL 'https://mycallbackurl.com/api/notify?validationtoken=NTk4NzliNzktNTE1MS00ZjE1LWJjNzctMTE4NzI4NTNhODlh' verification failed 'System.Net.WebException: The request was aborted: Could not create SSL/TLS secure channel.\r\n at System.Net.HttpWebRequest.GetResponse()\r\n at Microsoft.Exchange.Services.OData.ODataPushSubscriptionCallbackUrlValidationHelper.SendRequestAndVerifyResponse(Uri callbackUrl, PushSubscription pushSubscription)'."}
This is my reply that attempts to use that validation token:
REQUEST
https://mycallbackurl.com/api/notify?validationtoken=NTk4NzliNzktNTE1MS00ZjE1LWJjNzctMTE4NzI4NTNhODlh
RESPONSE CODE
200
RESPONSE BODY
NTk4NzliNzktNTE1MS00ZjE1LWJjNzctMTE4NzI4NTNhODlh
RESPONSE HEADERS
Date: Mon, 16 May 2016 09:42:50 GMT
Server: Apache-Coyote/1.1
Access-Control-Allow-Origin: chrome-extension://aejoelaoggembcahagimdiliamlcdmfm
Access-Control-Allow-Credentials: true
Access-Control-Allow-Headers: Authorization, Accept, Origin, Content-Type, Access-Control-Allow-Credentials, Access-Control-Allow-Origin
Access-Control-Expose-Headers: X-Api-Version
Access-Control-Max-Age: 3600
Access-Control-Allow-Methods: POST, PATCH, PUT, GET, OPTIONS, DELETE
Allow: POST, PATCH, PUT, GET, OPTIONS, DELETE
Cache-Control: no-cache, no-store, max-age=0, must-revalidate
Expires: 0
Pragma: no-cache
Strict-Transport-Security: max-age=31536000 ; includeSubDomains
X-Content-Type-Options: nosniff
X-Frame-Options: DENY
X-XSS-Protection: 1; mode=block
X-Api-Version: 1.3
Content-Type: text/plain;charset=UTF-8
Content-Length: 48
Keep-Alive: timeout=5, max=100
Connection: Keep-Alive
The only think that changes between what is expected for and what I am providing is content type:
Expected: text/plain
Produced: text/plain;charset=UTF-8
Assuming this is the reason of the error, would not make sense to allow both cases?

Answer 1

The error indicates a failure to establish a secure connection (SSL). There may be a problem with your SSL certificate. Is it a self-issued cert perhaps?