สอบถามเรื่อง error ครับ The message received from the server cloud not be parsed.

<asp:UpdatePanel ID="upnPopupWarning" runat="server">
      <ContentTemplate>
                            <asp:Button ID="mstBtnPopupWarning" Text="Show Modal Popup" Style="display: none;"
                                runat="server" UseSubmitBehavior="false" CausesValidation="false" />
                            <ajaxToolkit:ModalPopupExtender ID="mstModalPopupExtenderWarning" BackgroundCssClass="modalBackground"
                                PopupControlID="pnlMessageBoxWarning" TargetControlID="mstBtnPopupWarning" DropShadow="True"
                                PopupDragHandleControlID="pnlDragPopupWarning" OkControlID="btnWarningOK" CancelControlID="btnWarningCancel"
                                runat="server" BehaviorID="programmaticModalPopupBehavior">
                            </ajaxToolkit:ModalPopupExtender>
                            <asp:Panel ID="pnlMessageBoxWarning" CssClass="modalPopup" Style="display: none;"
                                Width="80%" runat="server">
                                <asp:Panel ID="pnlDragPopupWarning" Style="line-height: 30px; padding-left: 10px;
                                    background-color: Orange; vertical-align: middle; cursor: move;" runat="server">
                                    <asp:Label ID="lblPopupWarningHeading" runat="server">
                                    </asp:Label>
                                    <asp:ImageButton ID="imgClosePopupWarning" ImageUrl="~/Images/close02.png" Style="float: right;
                                        display: inline; cursor: pointer;" runat="server" ToolTip="Close" />
                                </asp:Panel>
                                <div id="msgContent" style="width: 100%; padding: 20px;">
                                    <asp:Literal ID="ltrContentMessageWarning" runat="server"></asp:Literal>
                                </div>
                                <br />
                                <div style="line-height: 30px; text-align: right; padding-right: 16px; vertical-align: middle;">
                                    <asp:Button ID="btnWarningOK" Text="" Visible="true" Width="100px" runat="server"
                                        Style="width: 100px; cursor: pointer;" />
                                   
                                        <asp:Button ID="btnWarningCancel" Text="" Visible="true" Style="width: 100px; margin-left: 4px;
                                            cursor: pointer;" runat="server" />
                                    
                                </div>
                                <br style="line-height: 8px;" />
                            </asp:Panel>
      </ContentTemplate>
</asp:UpdatePanel>