https://wpforo.com/community/how-to-and-troubleshooting-2/how-to-use-recaptcha-net-instead-of-google-com-in-wpforos-recaptcha-integration/

@ktalos

use hCaptcha for WordPress 😉

This is the error that I see, and checking the box doesn't work. hCaptcha works perfectly on the rest of the site.

UPDATE: 
I was able to get hCaptcha working with wpForo. It was actually simple to do because hCaptcha is a drop-in replacement for reCaptcha. However, since wpForo is hardcoded for reCaptcha, I had to manually edit wpForo to get it working. The problem with this is that every time wpForo is updated, I will have to go back in and re-edit whenever I run the update for wpForo.

The wpForo team writes fantastic plugins. After going through the code myself, I am convinced it would only take them minutes to add this functionality alongside reCaptcha. It would be a win-win situation for everyone and site owners would then be able to simply choose between the two of them. Please add hCaptcha functionality to the official plugin!