I suspect that you haven't set up the correct mailer setting from your site backend configuration page so when the system tries to send an email to the user, then it shows this error "Could not instantiate mail function.".

Can you try to turn off this "Send E-mails During Page Load" from the backend > Easyblog > setting > notification > general > Cronjob setting

If the issue still persists, can you edit your first post and update your site backend and FTP access so we can investigate this issue directly on your site?