Sharepoint - Failed to validate notification URL in /subscriptions endpoint

I'm having trouble registering webhooks on document libraries using the /subscriptions endpoint.

When I use ngrok as the noticafication URL on my local environment, I am able to create the webhooks and listen to events successfully. However, after deploying to the dev environment and using an AWS API gateway (public)endpoint as a notification URL, I get the following error message:

Failed to validate the notification URL 'https:/***********/webhook/sharepoint'

Interestingly, when I hit the same API from Postman, it works fine. The endpoint is active and returns the validation token in less than 5 seconds.

I also tested the endpoints using the SSL server test tool and confirmed the endpoint is set to TLS 1.2.

What else could be the potential issue here?

I had a similar problem. Only on the Azure platform (WebApi .NET6).

The tutorial example (Azure Function) worked. But when I flipped it to WebApi I got a validation error.

It helped me first: MS Documentation

I found that the Content-Type is not being sent.

When I tried a request through Postman without a Content-Type, I got a 415 Unsupported Media Type from my WebApi.

I turned on web server logging in Azure. I ran the webhook registration (Add-PnPWebhookSubsciption). And I got the same error.

And that was the point. I modified the WebApi method to not have any [FromBody]/[FromQuery] input parameters and parsed them directly in the method body.

Stackoverflow 415

Handling 415

Registration was successful.

The error may not be the same, but this procedure may be helpful in finding the cause.

[AllowAnonymous]
[HttpPost]
[Route("webhookRequest")]
public async Task<IActionResult> WebhookRequest()
{
    string validationToken = Request.Query["validationtoken"];

    if (validationToken != null)
    {
        return (ActionResult)new OkObjectResult(validationToken);
    }

    string requestBody = await new StreamReader(Request.Body).ReadToEndAsync();
    var data = JsonConvert.DeserializeObject<Response<Notification>>(requestBody);
    // ....
}