Invalid postback or callback argument. Event validation is enabled using in configuration

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C#

Hi
I dont know where i'm going wrong.

Because i have a button click vent in my code.When ever i click on the button it throws the above error.

Here is my code(button click):

protected void btnupdate_Click(object sender, EventArgs e)
        {
            if (TXTForwarder.Text == "" || TXTPODG.Text == "" || TXTPOLG.Text == "" || TXTShippingLine.Text == "" || TXTContainerType.Text == "" || TXTContainerSize.Text == "" || TXTValidFrom.Text == "" || TXTValidTo.Text == "" || TXTBasicRate.Text == "" || TXTPAF.Text == "" || TXTCAF.Text == "" || TXTTotalAmount.Text == "" || TXTFreeDays.Text == "" || TXTCreditDays.Text == "" || TXTNITDeposit.Text == "" || TXTForwarderReference.Text == "")
            {
                Page.SetFocus(TXTPOLG);
            }
            else
            {
                    string flaG = "U";
                    add(flaG);
            }
        }

Here is my revelant aspx code:

<asp:UpdatePanel ID="UpControls" runat="server" UpdateMode="Conditional">
               <ContentTemplate>
                   <asp:Panel ID="Panel3" runat="server" Visible = "true" >
                       <table>
                       <tr>
                            <td class="label">
                               <asp:Label ID="lblPOL" runat="server" Text="" />
                           </td>
                           <td>
                               <asp:TextBox ID="TXTPOLG" runat="server" CssClass="HtmlTxt"  Width="200px"></asp:TextBox>
                           </td>
                           <td class="label">
                               <asp:Label ID="lblPOD" runat="server" Text="" />
                           </td>
                           <td>
                               <asp:TextBox ID="TXTPODG" runat="server" CssClass="HtmlTxt"  Width="200px"></asp:TextBox>
                           </td>
                       </tr>
                       <tr>
                            <td class="label">
                               <asp:Label ID="lblForwarder" runat="server" Text="" />
                           </td>
                           <td>
                               <asp:TextBox ID="TXTForwarder" runat="server" CssClass="HtmlTxt"  Width="200px"></asp:TextBox>
                           </td>
                            <td class="label">
                               <asp:Label ID="lblForwarderReference" runat="server" Text="" />
                           </td>
                           <td>
                               <asp:TextBox ID="TXTForwarderReference" runat="server" CssClass="HtmlTxt"  Width="200px"></asp:TextBox>
                           </td>
                        </tr>
                        <tr>
                            <td class="label">
                               <asp:Label ID="lblShipping" runat="server" Text="" />
                           </td>
                           <td>
                               <asp:TextBox ID="TXTShippingLine" runat="server" CssClass="HtmlTxt"  Width="200px"></asp:TextBox>
                           </td>
                            <td class="label">
                               <asp:Label ID="lblContainerType" runat="server" Text="" />
                           </td>
                           <td>
                               <asp:TextBox ID="TXTContainerType" runat="server" CssClass="HtmlTxt"  Width="200px"></asp:TextBox>
                           </td>
                        </tr>
                        <tr>
                            <td class="label">
                               <asp:Label ID="lblContainerSize" runat="server" Text="" />
                           </td>
                           <td>
                               <asp:TextBox ID="TXTContainerSize" runat="server" CssClass="HtmlTxt"  Width="200px"></asp:TextBox>
                           </td>
                            <td class="label">
                               <asp:Label ID="lblValidFrom" runat="server" Text="" />
                           </td>
                           <td>
                               <asp:TextBox ID="TXTValidFrom" runat="server" CssClass="HtmlTxt"  Width="200px"></asp:TextBox>
                           </td>
                        </tr>
                        <tr>
                            <td class="label">
                               <asp:Label ID="lblValidTo" runat="server" Text="" />
                           </td>
                           <td>
                               <asp:TextBox ID="TXTValidTo" runat="server" CssClass="HtmlTxt"  Width="200px"></asp:TextBox>
                           </td>
                            <td class="label">
                               <asp:Label ID="lblBasicRate" runat="server" Text="" />
                           </td>
                           <td>
                               <asp:TextBox ID="TXTBasicRate" runat="server" CssClass="HtmlTxt"  Width="200px"></asp:TextBox>
                           </td>
                        </tr>
                        <tr>
                            <td class="label">
                               <asp:Label ID="lblPAF" runat="server" Text="" />
                           </td>
                           <td>
                               <asp:TextBox ID="TXTPAF" runat="server" CssClass="HtmlTxt"  Width="200px"></asp:TextBox>
                           </td>
                            <td class="label">
                               <asp:Label ID="lblCAF" runat="server" Text="" />
                           </td>
                           <td>
                               <asp:TextBox ID="TXTCAF" runat="server" CssClass="HtmlTxt" Width="200px" ></asp:TextBox>
                           </td>
                        </tr>
                        <tr>
                          <td class="label">
                               <asp:Label ID="lblPSS" runat="server" Text="" />
                           </td>
                           <td>
                               <asp:TextBox ID="TXTPSS" runat="server" CssClass="HtmlTxt"  Width="200px"></asp:TextBox>
                           </td>
                            <td class="label">
                               <asp:Label ID="lblTotalAmount" runat="server" Text="" />
                           </td>
                           <td>
                               <asp:TextBox ID="TXTTotalAmount" runat="server" CssClass="HtmlTxt"  Width="200px"></asp:TextBox>
                           </td>
                        </tr>
                        <tr>
                            <td class="label">
                               <asp:Label ID="lblFreeDays" runat="server" Text="" />
                           </td>
                           <td>
                               <asp:TextBox ID="TXTFreeDays" runat="server" CssClass="HtmlTxt"  Width="200px"></asp:TextBox>
                           </td>
                            <td class="label">
                               <asp:Label ID="lblCreditDays" runat="server" Text="" />
                           </td>
                           <td>
                               <asp:TextBox ID="TXTCreditDays" runat="server" CssClass="HtmlTxt"  Width="200px"></asp:TextBox>
                           </td>
                        </tr>
                        <tr>
                            <td class="label">
                               <asp:Label ID="lblNITDeposit" runat="server" Text="" />
                           </td>
                           <td>
                               <asp:TextBox ID="TXTNITDeposit" runat="server" CssClass="HtmlTxt"  Width="200px"></asp:TextBox>
                           </td>
                           <td class="field" align="right" width="171">
                               <asp:Button CssClass="formbutton" ID="btnInsert" runat="server" TabIndex="9" Text="Insert"
                               OnClick="btnadd_Click"></asp:Button>
                           </td>
                           <td class="field" align="right" width="171">
                               <asp:Button CssClass="formbutton" ID="btnUpdate" runat="server" TabIndex="9" Text="Insert"
                               OnClick="btnupdate_Click"></asp:Button>
                           </td>
                        </tr>
               </table>
          </asp:Panel>


          </ContentTemplate>
          </asp:UpdatePanel>

Please help help :(

Thanks in advance.