Re are receiving this error message from recaptcha (which is not showing on registration) - see screenshot attached. We ruled out the K2 Recaptcha conflict. Problem exists in MiniRegistration as well as full registration forms.
PS: As the site is productive and live we had to remove the Recaptcha field temporarily and set user accounts to manual approval by admin.
It seems that your current template may caused this issues to happens. I already tried with our theme that "protostar", and work fine as shown http://prntscr.com/c3wnnt. Please contact your current(JSN Glass PRO - Default) template provider regarding this issues.