If you are getting a Fatal error like
Fatal error: Call to undefined function mb_convert_case()
The function mb_convert_case() shown in the above error is a predefined php function which is not included in your php server settings. Please ask your hosting team to enable this (mbstring extension) to fix the issue.
Just paste the below code in a php file called info.php and upload it in the root your server files and run this url http://yourdomain.com/info.php and you can view your server variables. Change yourdomain.com to your domain name. Search for mbstring and you can make sure it is installed or not.
<?php phpinfo(); ?>