求助：用jQuery Ajax 上传图片显示“上传失败，错误信息：SyntaxError:语法错误
问题，如标题所述，有时显示“上传失败，错误信息：SyntaxError:拒绝访问”.

代码如下：（HTML页）

<script type="text/javascript" src="../js/singleupload.js"></script>
<script type="text/javascript" src="../js/jquery-1.3.2.min.js"></script>
<script type="text/javascript" src="../js/jquery.validate.min.js"></script>


 <asp:TextBox ID="txtImgUrl" runat="server" CssClass="input w380 left"></asp:TextBox>
  <a href="javascript:void(0);" class="files"><input type="file" id="FileUpload" name="FileUpload" /></a>
  <span class="uploading">正在上传，请稍候...</span>


（singleupload.js页）

$(function() {
  $("#FileUpload").bind("change", function() {
  //开始提交
  $("#form1").ajaxSubmit({
  beforeSubmit: function(formData, jqForm, options){
  //隐藏上传按钮
  $(".files").hide();
  //显示LOADING图片
  $(".uploading").show();
  },
  success: function(data, textStatus) {
  if (data.msg == 1) {
  $("#txtImgUrl").val(data.msbox);
  } else {
  alert(data.msbox);
  }
  $(".files").show();
  $(".uploading").hide();
  },
  error: function(data, status, e) {
  alert("上传失败，错误信息：" + e);
  $(".files").show();
  $(".uploading").hide();
  },
  url: "../../Tools/SingleUpload.ashx",
  type: "post",
  dataType: "json",
  timeout: 600000
  });
  });
});



（SingleUpload.ashx 页)


namespace SingleUpload
{
  /// <summary>
  /// AJAX单文件上传页
  /// </summary>
  public class SingleUpload : IHttpHandler, IRequiresSessionState
  {

  public void ProcessRequest(HttpContext context)
  {
  //检查是否登录后上传操作
  if (context.Session["UserId"] == null)
  {
  context.Response.Write("{msg: 0, msbox: \"请登录后再进行上传文件！\"}");
  return;
  }
  HttpPostedFile _upfile = context.Request.Files["FileUpload"];
  string _delfile = context.Request.Params["txtImgUrl"];
  if (_upfile == null)
  {
  context.Response.Write("{msg: 0, msbox: \"请选择要上传文件！\"}");
  return;
  }
  UpLoad upFiles = new UpLoad();
  string msg = upFiles.fileSaveAs(_upfile, 0);
  //删除已存在的旧文件
  if (!string.IsNullOrEmpty(_delfile))
  {
  string _filename = HttpContext.Current.Server.MapPath(_delfile);
  if (File.Exists(_filename))
  {
  File.Delete(_filename);
  }
  }