Okay, I have checked your admin panel, issue is from your phpmyadmin ( sql_mode ) Because in your hosting have some restriction that's the reason for not creating users and not working something. Go to phpmyadmin and Remove STRICT_TRANS_TABLES from sql mode variable after that your admin panel will work fine. and also app will work fine.
You can do by itself if you have root access. or you can contact with your hosting provider for this.