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ASP.Net AjaxControlToolkit AsyncFileUpload in update panel that is created in server-side code with master page c#
<form id="form1" runat="server" enctype="multipart/form-data" method="post">

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>

1.  private Control _generateControl(FormFields formFieldObject, string questionID) {
 2.         logger.Debug("start of method _generateControl()");
 3.         Control genericControl = _formBLL.getControlForForm(formFieldObject);
 4.         Type typeofcontrol = genericControl.GetType();
 5.         logger.Debug(typeofcontrol.ToString());
 6.
 7.         HtmlGenericControl controlHolder = new HtmlGenericControl("div");
 8.         controlHolder.ID = _ns + "formfieldHolder_" + formFieldObject.id;
 9.         controlHolder.Attributes.Add("class", _ns + "formfieldHolder");
 10.
 11.        if (typeofcontrol == typeof(AsyncFileUpload))
 12         {
 13.            #region display question number and question
 14.            Label questionIDLabel = new Label();
 15.            questionIDLabel.CssClass = _ns + "questionIDLabel";
 16.            questionIDLabel.Text = questionID;
 17.            Label questionText = new Label();
 18.            questionText.CssClass = _ns + "questionText";
 19.            questionText.Text = formFieldObject.name;
 20.            #endregion
 21.
 22.            #region add holder for file upload control
 23.            HtmlGenericControl questionControlHolder = new HtmlGenericControl("div");
 24.            questionControlHolder.Attributes.Add("class", _ns + "questionControlHolder");
 25.            genericControl.ID = _ns + "questionControl_" + formFieldObject.id;
 26.            #endregion
 27.
 28.            AsyncFileUpload fuc = (AsyncFileUpload)genericControl;
 29.            fuc.ID = _ns + "questionControl_" + formFieldObject.id;
 30.            fuc.ClientIDMode = ClientIDMode.AutoID;
 31.            fuc.UploadedComplete += fileuploadClick;
 32.            fuc.UploadedFileError += fileuploadError;
 33.            questionControlHolder.Controls.Add(fuc);
 34.
 35.            // add the controls to the holder
 36.            #region add controls to page
 37.            controlHolder.Controls.Add(questionIDLabel);
 38.            controlHolder.Controls.Add(questionText);
 39.            controlHolder.Controls.Add(questionControlHolder);
 40.            #endregion
 41.        }
 42.        return controlHolder;
 43.  }

<asp:UpdatePanel ID="questionPanel" runat="server" UpdateMode="Conditional">
    <ContentTemplate>
        <asp:PlaceHolder ID="QuestionSection" runat="server">
        </asp:PlaceHolder>
    </ContentTemplate>
</asp:UpdatePanel>

public void fileuploadError(object sender, AsyncFileUploadEventArgs e)
    {
        logger.Debug("START method fileuploadError()");
        logger.Debug("END method fileuploadError()");
    }
    public void fileuploadClick(object sender, AsyncFileUploadEventArgs e)
    {
        logger.Debug("START method fileuploadClick()");
        logger.Debug("END method fileuploadClick()");
    }

Server Response Error: 'Unknown Server Error'
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Unhandled Exception: Server Response Error: 'Unknown Server Error'

Index or size is negative or greater than the allowed amount
 [Break On This Error] Type.registerNamespace("Sys.Extended.U...FileUploadEventArgs",Sys.EventArgs);

public void Page_Init(object sender, EventArgs e)
    {
        logger.Debug("Page_Init START");
        if (!Page.IsPostBack) { Session["qid"] = 0; }
        int qid = Convert.ToInt32(Session["qid"]);
        if (qid > 0)
        {
            _displaySingleQuestion(_formObject, qid);
        }
        logger.Debug("Page_Init END");
    }

if (!Page.IsPostBack) { Session["qid"] = 0; }