when uploading picture it redirect to login page

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ASP.NET

i am using session and very working fine but when i upload the picture and press button it redirect to login.aspx...
dont know why this is happing

protected void Page_Load(object sender, EventArgs e)
      {


          string username = (string)(Session["UserAuthentication"]);
              if((Session["UserAuthentication"]!=null) && (!Page.IsPostBack))
              {
                  Label2.Text=username;

                string query ="SELECT Node,Title, Source, Scope, Criticality, Vendor, Dependancy, Status, [Start Week], ID, Resource FROM ['2013 Projects$'] WHERE Resource='" + username + "'";
                da = new SqlDataAdapter(query, connstring);
                dt = new DataTable();
                da.Fill(dt);
                GridView3.DataSource = dt;
                GridView3.DataBind();

              }
          else
              {
                  Response.Redirect("Login.aspx");
              }
          if (!Page.IsPostBack)
          {
              //data();
              tasks();

          }
      }

protected void Button2_Click1(object sender, EventArgs e)
       {
           StartUpLoad();
       }
       private void StartUpLoad()
       {
           //get the file name of the posted image
           //   string username = (string)(Session["UserAuthentication"]);

              string imgName = "default.jpg";
           //sets the image path
           string imgPath = "Images/gallery/adil/" + imgName;

           //get the size in bytes that

           int imgSize = FileUpload1.PostedFile.ContentLength;

           //validates the posted file before saving
           if (FileUpload1.PostedFile != null && FileUpload1.PostedFile.FileName != "")
           {
               // 10240 KB means 10MB, You can change the value based on your requirement
               if (FileUpload1.PostedFile.ContentLength > 102400)
               {
                   Page.ClientScript.RegisterClientScriptBlock(typeof(Page), "Alert", "alert('File is too big.')", true);

               }
               else
               {
                   //then save it to the Folder
                   FileUpload1.SaveAs(Server.MapPath(imgPath));
                   Image1.ImageUrl = "~//" + imgPath;
                   Page.ClientScript.RegisterClientScriptBlock(typeof(Page), "Alert", "alert('Image saved!')", true);

               }

           }


       }