UpdatePanel-hosted TextBox is not sent via button click

Hi,

I've got a .NET 4.0 WebForms project with 2 UpdatePanels and textbox in one of them. When I enter text in "Group" textbox and then click on "Save ASYNC" button, the proper server function is called but nothing comes to txtGroup. I wonder what's wrong with my project and how to fix the issue. Below is my ASPX page:

<%@ Page Title="Home Page" Language="C#" MasterPageFile="~/DEF.Master" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="WebApplication1._Default" %>


<asp:Content ID="Content2" ContentPlaceHolderID="PlaceHolderBodyAreaClass" runat="server">
        <link href="Content/css/jquery-ui.css" rel="stylesheet" type="text/css" />
        <link href="Content/CSS/AdminStyles.css" rel="stylesheet" type="text/css" />
</asp:Content>

<asp:Content ID="Content1" ContentPlaceHolderID="PlaceHolderAdditionalPageHead" runat="server">

    <script type="text/javascript" src="Scripts/js/jquery-1.11.2.js"></script>
    <script type="text/javascript" src="Scripts/js/jquery-ui.js"></script>

</asp:Content>


<asp:Content ID="BodyContent" ContentPlaceHolderID="PlaceHolderMain" runat="server">

    <asp:UpdatePanel ID="UpdatePanel4" runat="server"  >
        <ContentTemplate>
            <asp:Label ID="lblError" runat="server"  ForeColor="Red" Text=""></asp:Label>
            <asp:Button ClientIDMode="Static" ID="Button1" runat="server" Text="DIALOG"
                OnClientClick="Dlg();return false;"
                />
        </ContentTemplate>
    </asp:UpdatePanel>

    <div id="dialogQuests" class="dialogQuestsClass">
        <div class="headerDivBlock">QUESTION</div>
        <div class="contentDivBlock">
            <asp:UpdatePanel ID="UpdatePanel6" runat="server" >
                <ContentTemplate>
                    <table id="tableSizePositionQuest">
                        <tr>
                            <td>Group:</td>
                            <td>
                                <asp:TextBox ID="txtGroup" runat="server"></asp:TextBox>
                            </td>
                        </tr>
                        <tr>
                            <td colspan="2">
                                <asp:Button ID="btnQuestSave" runat="server" UseSubmitBehavior="false" Text="Save ASYNC" OnClick="btnQuestSave_Click"  />
                            </td>
                        </tr>
                    </table>
                </ContentTemplate>
            </asp:UpdatePanel>
        </div>
    </div>


    <script type="text/javascript">

        function Dlg()
        {
            $('#dialogQuests').dialog({ minWidth: 840, resizable: true });
        }

    </script>

</asp:Content>