DennisO I assume it’s a bug in the hCaptcha extension. Unfortunately, even I don’t know what is going on exactly - maybe a necessary precondition to insert a new user is that the hCaptcha is present, which makes sense for the sign up forms. But, perhaps there is no exception to this in the admin form which doesn’t have the captcha?
Anyways, I have disabled the extension on your forum. If you can add the user successfully now, it’s definitely a bug in hCaptcha (or, incompatibility between hCaptcha and some other ext). I might look into this in more detail later when I have time
