I think you get this message because:
if ($database_connection_charset != 'utf8' && !extension_loaded('mbstring')) {
$result = "php_mbstring extension required";
}
In your previous message you get the following message error :
Fatal error: Call to undefined function mb_convert_encoding() in ..\modx\assets\snippets\AjaxSearch\AjaxSearch.php on line 67
because you try to use a mbstring function:
$searchString = mb_convert_encoding($_POST['search'],$pgEncoding , "UTF-8");
Could you check that your $database_connection_charset variable in the file /manager/includes/config.inc.php is initialized with "utf8". You should find a line :
$database_connection_charset = 'utf8';
Be carefull the true connection charset is 'utf8' not 'UTF-8'. Mysql base use 'utf8' and html pages 'UTF-8'

If this variable is not well initialized; Do it and reload your page in your browser
If yes, do an another test : open the AjaxSearch.php file and change the line 66 by :
$result = "AjaxSearch: php_mbstring extension required";
I think it's not necessary but i would like to be sure that your message come from the test of the line 65
Let me know the results