ASP.NET - Microsoft JScript runtime error: Sys.WebForms.PageRequestManagerServerErrorException: Invalid postback or callback argument
<asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional">
 <ContentTemplate>
  <asp:TextBox ID="TextVerifyName" runat="server" CssClass="input1" EnableViewState="false"></asp:TextBox>
  <asp:Button ID="ButtonVerifyName" runat="server" Text="Verify" CssClass="checkButton" onclick="ButtonVerifyName_Click" />
  <asp:UpdateProgress ID="UpdateProgressVerifyName" runat="server" AssociatedUpdatePanelID="UpdatePanel1">
   <ProgressTemplate>
    <img src="Graphics/spinner.gif" alt="Please wait" />
   </ProgressTemplate>
  </asp:UpdateProgress>
 </ContentTemplate>
</asp:UpdatePanel>

DropDownList1.Items.Add(new ListItem("Name", "Value"));

<asp:DropDownList ID="DropDownList1" runat="server" EnableViewState="false"></asp:DropDownList>