No, WP_DEBUG is set to false.
To avoid the displaying of errors (deprecated) in the ajax return, I finally replaced this line
require(‘../../../wp-config.php’);
by this block
ob_start();
require(‘../../../wp-config.php’);
ob_clean();
in the file mrp-search.php
And I remarked that there are a lot of deprecated errors in wp with php5.3.
Gilles