Cause
In the Webhook that you specified, a redirect is apparently executed. This causes the Microsoft IIS server to send a 411 Length required error message.

What causes this?
A redirect is executed on the Webhook URL that you specified to the Mollie API via Settings in your account or via the webhook URL parameter: the URL changes.

It usually concerns one of the following situations:

  • an HTTPS to HTTP redirect web address, or vice versa;
  • a domain or sub-domain redirect;
  • a website language redirect

Check whether this also occurs with your own Webhook URL, simply by visiting it in the web browser. Pay close attention to how the URL/the web address changes.

Solution
The 411 Length required error can be solved in 2 different ways:

  • Specify the eventual URL after the redirect(s) as webhook URL;
  • Or specifically switch off the redirect(s) for the webhook.