Hai Graham,

I've checked your backend admin site and found out that you using Recaptcha in you registration form but at you front end site register form don't have any Recaptcha indicator, that why you hitting an error.

i have remove Recaptcha in you registration form, can you give it a check now and try to register demo user.

By the way if you want to have Recaptcha in you form you can read our documentation at http://stackideas.com/docs/easysocial/administrators/applications/recaptcha-v2