I switched reCAPTCHA from v2 to v3 and now my forms fail. What went wrong?

Google reCAPTCHA keys belong to one version, so v2 keys do not work with v3 and the other way round. Create new keys in Google's reCAPTCHA admin for the version you want, and add your store domain there. Then open Plugins, then Recaptcha, set Version / type, paste the new Site key and Secret key, and click Save. Test a login and a form on your storefront afterwards.