Hey @paweljurczynski,
We usually return a 303 when there is an issue with the reCAPTCHA. Are you using a custom reCAPTCHA? If so, can you confirm that it is configured to work with your domain (you’d do that in your Google dashboard)?
If not, can you please share your form code? It could be that you set data-netlify-recaptcha="true" but then do not create the necessary div with the reCAPTCHA in the body of your form.