Requested Json parse failed in ajax

i have this error “Requested Json parse failed” in (ajax), i had a look at response data and it’s OK, my datatype is “json” and setContentType is (“application/json”)…what could it be?

This is ma ajax call…
`

function saveValues(){
    var cab=$("#cab").val();
    var description=$("#description").val();    
    var cabret=$("#cabret").val();
    $.ajax({
        type: "POST",
        url: "save.html",
        data: {"abi" : abi,"cab" : cab,"description" : description,"cabret":cabret},
        dataType: "json",
        success: function(json){
                    if(json=="OK"){
                        alert("<spring:message code='msg.ok.operation' />");
                    }
                    else{
                        alert("<spring:message code='msg.error.operation' />");
                    }
                 },
                 error: function(jqXHR, exception) {
                     if (jqXHR.status === 0) {
                         alert('Not connect.n Verify Network.');
                     } else if (jqXHR.status == 404) {
                         alert('Requested page not found. [404]');
                     } else if (jqXHR.status == 500) {
                         alert('Internal Server Error [500].');
                     } else if (exception === 'parsererror') {
                         alert('Requested JSON parse failed.');
                     } else if (exception === 'timeout') {
                         alert('Time out error.');
                     } else if (exception === 'abort') {
                         alert('Ajax request aborted.');
                     } else {
                         alert('Uncaught Error.n' + jqXHR.responseText);
                     }
                 }
    });

`