返回HTTP 302的Ajax调用

Question

When we make this ajax call, intermittently I get HTTP status code as 302. While most of the times, the ajax call is successful as well returning HTTP status as 200 OK. I am unable to get the exact reason for this and how to overcome this scenario. The HTTP status 302 is returned in particular scenario when I leave the application idle for 10-15 minutes and then make this call. However when I hit the URL directly in browser the correct data is returned. Please advice.

function checkProgressStatus(event){            
     $.ajax({
             url: "/"+window.AppContext+"/servlet/Progress",
             cache: false,
             dataType: 'json',
             async:false,
             success: function(data,ev){ 
                 if(data.Progress == 'INPROGRESS'){                     
                    alert("Execution in progress");
                    event.stopImmediatePropagation();                       
                 }

             },
        error: function (exception) {
            alert("Request to check inprogress status failed.");
            window.open("/"+window.AppContext+"/path/filter/AnotherPage.html");
        }
     });
 }