Perhaps the problem is with apache mod_security. Try ajax GET instead of POST :
jQuery.ajax({ type:"GET", url: "cart_ajax_get_product.php", data: {id: 355, qty: 1}, success: function(data) { }); }, error: function(err) { } });
Or if this does not help ...
You can try to set these parameters on the .htaccess server or configure them elsewhere:
SecFilterScanPOST Off SecFilterEngine Off
source share