Option 1: If you have access to your php.ini file, set this mbstring.func_overload = 0
It's a requirement of PHPExcel, in their Autoloader.php file:
...
if (ini_get('mbstring.func_overload') & 2) {
throw new PHPExcel_Exception('Multibyte function overloading in PHP must be disabled for string functions (2).');
}
...
Option 2: If you don't have privileges to edit php.ini, you can setup this in an .htaccess file:
php_value mbstring.func_overload 0