I'm getting this error like An extender can't be in a different UpdatePanel than the control it extends. i'm wondering what's wrong with my html markup :
<asp:Panel ID="Panel_AddNew" runat="server">
<asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional">
<ContentTemplate>
<asp:HiddenField ID="hd" runat="server" />
<asp:HiddenField ID="template_id" runat="server" />
<table class="table text-center no-space table-bordered" style="vertical-align: middle; text-align: left;">
<tr>
<td style="width: 20%">Title
</td>
<td>
<asp:TextBox ID="txt_ComName1" runat="server" CssClass="form-control" ValidationGroup="AddNewMail" name="txt_ComName1"></asp:TextBox>
</td>
</tr>
<tr>
<td colspan="2">
<asp:UpdatePanel ID="UpdatePanel8" runat="server" UpdateMode="Conditional">
<ContentTemplate>
<table>
<tr>
<td style="width: 29%">Subscribers Category
</td>
<td>
<asp:DropDownList ID="ddl_subscribercategories" Width="300px" Font-Bold="false" CssClass="form-control" runat="server" AutoPostBack="True" OnSelectedIndexChanged="ddl_subscribercategories_SelectedIndexChanged">
</asp:DropDownList>
</td>
</tr>
<tr>
<td style="width: 29%">From User </td>
<td>
<table>
<tr>
<td>
<asp:TextBox ID="txt_ComName2" runat="server" CssClass="form-control" ValidationGroup="AddNewMail" Width="500px" name="txt_ComName2"></asp:TextBox>
</td>
<td> </td>
<td>
<asp:ImageButton ID="Img5" runat="server" Height="30px" ImageUrl="~/img/1398451597_Table.png" ToolTip="Select From User" Width="25px" />
<ajaxToolkit:ModalPopupExtender ID="MPE1" runat="server" BackgroundCssClass="modal-backdrop fade in" CancelControlID="btnclose" Enabled="True" PopupControlID="Panel_FromUser_master" TargetControlID="Img5">
</ajaxToolkit:ModalPopupExtender>
</td>
</tr>
</table>
</td>
</tr>
<tr>
<td style="width: 29%">To User </td>
<td>
<table>
<tr>
<td>
<asp:TextBox ID="txt_ComName3" runat="server" CssClass="form-control" ValidationGroup="AddNewMail" Width="500px" name="txt_ComName3"></asp:TextBox>
</td>
<td> </td>
<td>
<asp:ImageButton ID="Img2" runat="server" Height="30px" ImageUrl="~/img/1398451597_Table.png" ToolTip="Select To User Subscribers" Width="25px" OnClick="Img2_Click" />
<ajaxToolkit:ModalPopupExtender ID="MPE" runat="server" BackgroundCssClass="modal-backdrop fade in" BehaviorID="mPopup" CancelControlID="ibtnclose" DynamicServicePath="" Enabled="True" PopupControlID="Panel_Subscriber_Master" TargetControlID="hd">
</ajaxToolkit:ModalPopupExtender>
</td>
</tr>
</table>
</td>
</tr>
<tr>
<td style="width: 29%">Cc </td>
<td>
<table>
<tr>
<td>
<asp:TextBox ID="txt_ComName4" runat="server" CssClass="form-control" ValidationGroup="AddNewMail" Width="500px" name="txt_ComName4"></asp:TextBox>
</td>
<td> </td>
<td>
<asp:ImageButton ID="Img3" runat="server" Height="30px" ImageUrl="~/img/1398451597_Table.png" ToolTip="Select Cc Subscribers" Width="25px" OnClick="Img3_Click" />
</td>
</tr>
</table>
</td>
</tr>
<tr>
<td style="width: 29%">Bcc </td>
<td>
<table>
<tr>
<td>
<asp:TextBox ID="txt_ComName5" name="txt_ComName5" runat="server" Columns="10" CssClass="form-control" Rows="8" TextMode="MultiLine" ValidationGroup="AddNewMail" Width="500px"></asp:TextBox>
</td>
<td> </td>
<td>
<asp:ImageButton ID="Img4" runat="server" Height="30px" ImageUrl="~/img/1398451597_Table.png" ToolTip="Select Bcc Subscribers" Width="25px" OnClick="Img4_Click" />
</td>
<td> </td>
<td>
<asp:ImageButton ID="Img6" runat="server" Height="28px" ImageUrl="~/img/20100615143456!Icon-template.png" ToolTip="Select Template" Width="23px" />
<ajaxToolkit:ModalPopupExtender ID="MPE3" runat="server" BackgroundCssClass="modal-backdrop fade in" CancelControlID="ImageButton4" DynamicServicePath="" Enabled="True" PopupControlID="Panel_Template_master" TargetControlID="Img6">
</ajaxToolkit:ModalPopupExtender>
</td>
</tr>
</table>
</td>
</tr>
</table>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="ddl_subscribercategories" EventName="SelectedIndexChanged" />
<asp:AsyncPostBackTrigger ControlID="Img2" EventName="Click" />
<asp:AsyncPostBackTrigger ControlID="Img3" EventName="Click" />
<asp:AsyncPostBackTrigger ControlID="Img4" EventName="Click" />
</Triggers>
</asp:UpdatePanel>
</td>
</tr>
<tr>
<td style="width: 20%">Email Body </td>
<td>
<asp:UpdatePanel ID="UpdatePanel34" runat="server" UpdateMode="Conditional">
<ContentTemplate>
<CKEditor:CKEditorControl ID="CKEditor1" runat="server" PasteFromWordRemoveFontStyles="False" ValidationGroup="AddNewMail" name="CKEditor1"></CKEditor:CKEditorControl>
</ContentTemplate>
</asp:UpdatePanel>
<asp:CustomValidator ID="CustomValidator1" runat="server" ErrorMessage="Email body is required." ForeColor="Red" ValidationGroup="AddNewMail"></asp:CustomValidator>
</td>
</tr>
<tr>
<td style="width: 20%">Attach Files </td>
<td>
<asp:FileUpload ID="FileUpload1" runat="server" AllowMultiple="true" CssClass="multi" />
<br />
<asp:GridView ID="gv_file_list" runat="server" AutoGenerateColumns="False" DataKeyNames="Id" GridLines="None">
<Columns>
<asp:TemplateField>
<ItemTemplate>
<asp:Button ID="Button1" runat="server" BackColor="White" BorderColor="White" BorderStyle="None" CommandArgument='<%# Eval("Id") %>' CommandName="DeleteFile" CssClass="theme-font" Font-Underline="True" Text="x" />
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField>
<ItemTemplate>
<asp:Label ID="Label14" runat="server" Text='<%# Bind("File_Title") %>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>
<br />
<asp:Label ID="lblMessage" runat="server" />
</td>
</tr>
</table>
</div>
<div class="portlet-body bordered" style="text-align: right;">
<asp:Button ID="btn_saveAsdraft" runat="server" CssClass="btn blue"
OnClientClick="return validate();" ValidationGroup="AddNewMail" Text="Save As Draft" OnClick="btn_saveAsdraft_Click" />
<asp:Button ID="btn_save" runat="server" CssClass="btn green"
OnClientClick="return validate();" ValidationGroup="AddNewMail" Text="Submit" OnClick="btn_save_Click" />
<asp:Button ID="btn_cancel" runat="server" CssClass="btn red" Text="Cancel" CausesValidation="False" OnClick="btn_cancel_Click" />
</ContentTemplate>
<Triggers>
<asp:PostBackTrigger ControlID="btn_saveAsdraft" />
<asp:PostBackTrigger ControlID="btn_save" />
<asp:AsyncPostBackTrigger ControlID="btn_cancel" EventName="Click" />
</Triggers>
</asp:UpdatePanel>
</asp:Panel>
what was problem with this html markup?
Here is one control as hidden field hd inside update panel 1 just put it to update panel 8 resolve this problem.