reCaptcha validation ignored

I'm using Form v4.3.0 with v1.6.31 and reCaptcha Version 2 with checkbox on a modular page.
The Captcha ist working and i can send mails but if I do not check the reCapture checkbox and send the mail the 404-page is shown after pressing the send-button. Of course the email is not send. The .md is as follows.

form:
    name: my-nice-form
    action: /contact

    fields:

       ....
        - name: g-recaptcha-response:
          label: Captcha
          type: captcha
          recaptcha_not_validated: 'Captcha ist nicht korrekt!'
          validate:
            required: true
process:
        - captcha: true
          message: 'Successfully passed reCAPTCHA!'
        - email:...

Could anybody give me a hint, why the reCaptcha is ignored and the send-button is working without checking the reCaptcha field?
Thanks very much in advance.
PS: Problem /forum/forms-blueprints/recaptcha-validation-client-side-not-working-t2516 seems to be similiar perhaps, the JS is not working for me/not a solution.