Hello @achterlijk,
-->first, clear the cache of your browser then check if it works.
-->Please disable captcha then check if it works.
else
-->Please try to generate new keys and then use them. Make sure that the keys are not having any extra space at the end.
-->I ran into this issue once, and found that it was because I didn’t fill out the rest of the form containing the site name and domain name/URL, etc. then press the register button. More or less, this error means you used the keys, but didn’t submit the form to google so it can actually be used; scroll down and fill out the form and click Register.
-->if above does not work then try not to choose reCAPTCHA v3, but v2 instead. This should work fine. I guess sometimes it is not compatible with v3.
refer this link too:
https://developers.google.com/recaptcha/docs/faq
Thanks,