用extjs库的tree动态加载导航菜单，为什么第一次能加载成功，关闭页面后再次打开就无法加载成功？

用extjs库的tree动态加载导航菜单，第一次能加载成功，关闭页面后再次打开就无法加载成功，只能重新启动计算机后，第一次能加载成功，关闭页面后再次打开仍然无法加载成功
前台部分代码如下：
var root = new Ext.tree.AsyncTreeNode({
id : "0",
text : "后台管理系统",
loader : new Ext.tree.TreeLoader({
url : "Data/Main.aspx",
listeners : {
"beforeload" : function(treeloader, node) {
treeloader.baseParams = {
id : node.id,
method : 'POST'
};
}
}
})
});
// 创建树型面板
var menu = new Ext.tree.TreePanel({
id : "treePanel",
autoScroll : true,
animate : true,
root : root,
// 默认根目录不显示
rootVisible : false,
border : false,
animate : true,
lines : true,
enableDD : true,
containerScroll : true,
listeners : {
click : nodeClick
}
});
// 加载时自动展开根节点
menu.expandAll();

后台部分代码如下：
public partial class AdminMain : System.Web.UI.Page
  {
  public string jsons = string.Empty;
  protected void Page_Load(object sender, EventArgs e)
  {
  GetTreeData();
  Response.Write(jsons);
  }

  /// <summary>
  /// 获得树信息
  /// </summary>
  /// <param name="parentId"></param>
  /// <returns></returns>
  private void GetTreeData()
  {
  string id = Request.Form["id"];
  jsons = ListToJson(id);
  }

  private string ListToJson(string id)
  {
  string jsons = "";
  IList<SysFun> list = new List<SysFun>();
  List<TreeHelper> treeList = new List<TreeHelper>();
  if (Session["admin"] != null)
  {
  string admin = Session["admin"].ToString();
  int roleId = UserInfoManager.GetUserInfoByUserId(admin).UserRole.RoleId;
  if (id == null)
  {
  list = SysFunManager.GetAllSysFunsByRoleId(roleId, 0);
  foreach (SysFun sf in list)
  {
  TreeHelper th = new TreeHelper();
  th.id = sf.NodeId.ToString();
  th.leaf = SysFunManager.SysFunIsLeaf(sf.NodeId);
  th.iconCls = "icon" + sf.NodeId;
  th.text = sf.DisplayName;
  treeList.Add(th);
  }
  }
  else if (id != null)
  {
  int parentId = int.Parse(id);
  list = SysFunManager.GetAllSysFunsByRoleId(roleId, parentId);
  foreach (SysFun sf in list)
  {
  TreeHelper th = new TreeHelper();
  th.id = sf.NodeId.ToString();
  th.leaf = SysFunManager.SysFunIsLeaf(sf.NodeId);
  if (parentId == 0)
  {
  th.iconCls = "icon" + sf.NodeId;
  }
  else
  {
  th.iconCls = "menuicon";
  }
  th.text = sf.DisplayName;
  treeList.Add(th);
  }
  }
  else
  {
  jsons = "{success:false}";
  }
  jsons = JsonHelper.ToJSON(treeList);
   
  }
  else
  {
  jsons = "{success:false}";
  }

  return jsons;
  }
  }

CSS code

无法加载成功，只能重新启动计算机

重新启动iis服务器不行吗