Fatal error: Call to undefined function mb_strtolower() in..
The error points to the PHP function you have missing at your server configuration. The PHP mbstring extension, which is required to handle international character sets, is not available on your server. Check your PHP configuration and make sure that PHP has been compiled with --enable-mbstring.
In case your hosting provider cannot enable it, please submit a support ticket at https://themerex.ticksy.com and provide us with your FTP details: FTP hostname, username and password.