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We use Azure Functions and .NET 6.0.

Ok, so it looks like the latest change (only on sandbox for evaluation) has an impact on Azure. The change was that the validation-token is sent with the body is e.g.

{"validationToken":"f5520450-09d8-42b7-90d5-aa22282dccb8"}

And the content-length set with the length of the body. Can you show how the return of Validation-Token header is handled in your code?

Is it possible that Azure check the content-length != 0 and send the body only?

I could see the problem now.

In our code we checked if the content length was 0 and only then checked for the Validaten-Tken in the header. Because now the validationToken ist also send in the body - this code did not work anymore. When I change this it works again.

Note: Such a change may also impact other developers who may use a similar code as me!

Thanks for your help.

The validation token is sent only once when the subscription is being created. So you should not check the content-length header to decide if it is a validation post or not. Instead, you can just simply check if the validation-token header exists, then echo it and don't need to read the body. We showed that in all our example codes. E.g. in Node JS

var server = http.createServer(function(req, res) {
  if (req.method == 'POST') {
    if (req.url == "/webhook") {
      if (req.headers.hasOwnProperty("validation-token")) {
        res.setHeader('Content-type', 'application/json');
        res.setHeader('Validation-Token', req.headers['validation-token']);
        res.statusCode = 200;
        res.end();
      } else {
        var body = []
        req.on('data', function(chunk) {
          body.push(chunk);
        }).on('end', function() {
          body = Buffer.concat(body).toString();
          var jsonObj = JSON.parse(body)
          console.log(jsonObj.body);
        });
      }
    }
  } else {
    console.log("IGNORE OTHER METHODS")
  }
});