BAN or PURGE results to `501 Not Implemented`

Problem/Motivation

Whatever I set as Request Method BAN or PURGE I get the following in the logs when I execute drush p-queue-work.

purger_varnishbundled_83033cae2a: item failed due GuzzleHttp\Exception\ServerException, details (JSON): {"msg":"Server error: `BAN http:\/\/localhost:8080\/` resulted in a `501 Not Implemented` response: 501 Not Implemented<\/title> <\/head>
Not (truncated...) ","uri":"http:\/\/localhost:8080\/","method":"BAN","guzzle_opt":{"http_errors":true,"connect_timeout":1,"timeout":1},"headers":{"user-agent":"varnish_purger module for Drupal 8.","cache-tags" etc.

I don't see anything wrong in my varnish default.vcl:

  if (req.method == "PURGE") {
    # Check if the IP is allowed.
    # purge is the ACL defined at the begining
    if (!client.ip ~ purge) {
      # Return error code 405 (Forbidden) when not.
      return (synth(405, "This IP (" + client.ip + ") is not allowed to send PURGE requests."));
    }
    return (purge);
  }

  # Ban logic to remove multiple objects from the cache at once. Tailored to Drupal's cache invalidation mechanism
  # Only allow BAN requests from IP addresses in the 'purge' ACL.
  if (req.method == "BAN") {
      # Same ACL check as above:
      if (!client.ip ~ purge) {
          return (synth(403, "Not allowed."));
      }

      if (req.http.Purge-Cache-Tags) {
          ban("obj.http.Purge-Cache-Tags ~ " + req.http.Purge-Cache-Tags);
      }
      else {
          ban("obj.http.x-url ~ " + req.url + " && obj.http.x-host == " + req.http.host);
      }

      # Logic for the ban, using the Cache-Tags header. For more info
      # see https://github.com/geerlingguy/drupal-vm/issues/397.
      if (req.http.Cache-Tags) {
          ban("obj.http.Cache-Tags ~ " + req.http.Cache-Tags);
      }
      else {
       else {
          return (synth(403, "Cache-Tags header missing."));
      }

      # Throw a synthetic page so the request won't go to the backend.
      return (synth(200, "Ban added."));
  }

I tried all kind of settings but still blocked on this. Any help welcome.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes