As it is most likely the problem occurs because the mbstring PHP module is not active on your server, let's wait until we can verify that before looking into what other causes there could be.
To check, you can create a PHP file, call it phpinfo.php, with the following:
<?php
phpinfo( );
?>
Then use FTP to upload this file to the root directory of your site.
You can then visit yoursite.com/phpinfo.php which will pull up the phpinfo page.
Search the page content and you should be able to find whether mbstring is enabled (screenshot) or not.
Delete the phpinfo.php file when you are finished.
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