Hi,

Thanks for reply. First thing i always use IIS while develpoing web apps. And the problem is with DropDownLists postback. The DDLs are in UpdatePanel. And I have not used Response.Write() on this page. So this is not the cuase. The main problem is on my local iis everything is perfect.

I have read many articals/Links provided by u and noticed from the error that perhaps my request/reply is modified somewhere. because earlier I was using one UpdatePanel. And there are two DDLs which makes postBack. So I have changed my code and used two diffent UpdatePanels but the problem is still there. Some time problem arises sometime not. here is my code with update panels:

<tr>
            <td class="lefttd">
                Main City/State* :</td>
            <td>
               <asp:DropDownList ID="cityState" runat="server" CssClass="file_box"
                    Height="26px" OnSelectedIndexChanged="cityState_SelectedIndexChanged" AutoPostBack="true">
               <asp:ListItem Value="">--Select--</asp:ListItem>
                </asp:DropDownList></td>
        </tr>
        <tr>
            <td class="lefttd">
                City*:</td>
            <td>
            <asp:UpdatePanel ID="UpdatePanel2" runat="server">
             <ContentTemplate>
    
                <asp:DropDownList ID="city" runat="server"
                    OnSelectedIndexChanged="city_SelectedIndexChanged" AutoPostBack="true">
                <asp:ListItem Value="">--Select--</asp:ListItem>
                </asp:DropDownList>

                </ContentTemplate>
      <Triggers>
        <asp:AsyncPostBackTrigger ControlID="cityState" EventName="SelectedIndexChanged" />
      </Triggers>
   </asp:UpdatePanel>
                
            </td>
        </tr>
        <tr id="Tr4">
    <td class="lefttd">
         Deals In*:
         </td>
    
    <td> <table class="tblBorder0">
        <tr><td>
        <asp:UpdatePanel ID="UpdatePanel3" runat="server">
         <ContentTemplate>

        <asp:ListBox ID="locality" runat="server" Height="70px" Width="130px" ClientIDMode="Static" SelectionMode="Multiple">
            <asp:ListItem Value="">--Select--</asp:ListItem>
            </asp:ListBox>
            <asp:ListBox ID="localityHid" runat="server" Height="70px" Width="130px" ClientIDMode="Static" SelectionMode="Multiple" CssClass="hidden">
            <asp:ListItem Value="">--Select--</asp:ListItem>
            </asp:ListBox>

            </ContentTemplate>
      <Triggers>
        <asp:AsyncPostBackTrigger ControlID="city" EventName="SelectedIndexChanged" />
      </Triggers>
   </asp:UpdatePanel>

            </td>
</tr>

I applied so many changes some from yr provided links and some from net search but still stucked...