jQuery Ajax complete event response is null

Question

I am facing one issue while getting response from the server from Ajax call , response is coming null from the Ajax request this is happening to a particular user not for all users, I am making a CORS request and no error is coming on the other server data.responseJSON is coming null complete event is getting executed not error event can any one let me know what is the issue Thanks in advance.

  $.ajax({
            url: urlToUpdateTEID,
            data: JSON.stringify({ 
                "isNonDicomCase":self.IsNonDicomCase,
            }),
            // data: { "allCaseDetail": JSON.stringify(cases) },
            type: 'Post',
            contentType: 'application/json',
            complete: function (data) {

                LogMessage("NJUCommonFunctions UpdateMddxID Ajax call to rename file END " + data );

                if (data.responseJSON.success == true && data.responseJSON.UploadedCases != null && data.responseJSON.UploadedCases != '') {


                }  
            },
            error: function (jqXHR, exception) {

                LogMessage("NJUCommonFunctions UpdateMddxID  Ajax ERROR BLOCK jqXHR " +jqXHR + 'exception ' +exception);


            }
        });