Thanks for reaching out! The « invalid domain » error with reCAPTCHA typically means that the domain the form is being served from isn’t authorised in your Google reCAPTCHA settings.
Could you please add the domain sibforms.com to your Google reCAPTCHA settings? This needs to be done on the Google side (not in Brevo) — you can find it in your Google reCAPTCHA console under the settings for your key.