Validation of viewstate MAC failed

This error has just started today, it worked fine as recently as yesterday so I don't know if it was caused by a windows update (last night was weekly server maintenance night which includes updates).
This affects both the production server and my workstation where I develop.

The page loads fine but when I click the edit column on the gridview I get this error:
Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.

After Googling I found this page http://forums.asp.net/p/955145/1173230.aspx#1173230
I have done as suggested there which is to add  enableEventValidation="false" viewStateEncryptionMode ="Never" to the page directive but when I do that I get a new error:
Multiple controls with the same ID 'txtStartDate' were found. FindControl requires that controls have unique IDs.

Which is not the case! There is only one control named that, the only other place it is mentioned is for a validatorcontrol which points to this text box as the control to validate.
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QPRAsked:
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samtran0331Connect With a Mentor Commented:
>>I get a new error:
>>Multiple controls with the same ID 'txtStartDate' were found. FindControl requires that controls
>>have unique IDs.

In the above, the StartDate column and the ExpiryDate column both have textboxes named "txtStartDate" in their edititemtemplates....

try:

            <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" CellPadding="4"
                DataKeyNames="NoticeID" DataSourceID="SqlDataSource3" ForeColor="#333333" GridLines="None"
                Width="672px">
                <Columns>
                    <asp:CommandField ShowDeleteButton="True" ShowEditButton="True" />
                    <asp:BoundField DataField="NoticeID" HeaderText="NoticeID" InsertVisible="False"
                        ReadOnly="True" SortExpression="NoticeID" Visible="False" />
                    <asp:BoundField DataField="NoticeText" HeaderText="Notice" SortExpression="NoticeText" />
                    <asp:TemplateField HeaderText="StartDate">
                        <EditItemTemplate>
                            <asp:TextBox ID="txtStartDate" runat="server" Text='<%#Bind("StartDate", "{0:dd/MM/yyyy}") %>'>
                            </asp:TextBox>
                        </EditItemTemplate>
                        <ItemTemplate>
                            <asp:Label ID="lblStartDate" runat="server" Text='<%#Eval("StartDate", "{0:dd/MM/yyyy}") %>'>
                            </asp:Label>
                        </ItemTemplate>
                    </asp:TemplateField>
                    <asp:TemplateField HeaderText="ExpiryDate">
                        <EditItemTemplate>
                            <asp:TextBox ID="txtExpiryDate" runat="server" Text='<%#Bind("ExpiryDate", "{0:dd/MM/yyyy}") %>'>
                            </asp:TextBox>
                        </EditItemTemplate>
                        <ItemTemplate>
                            <asp:Label ID="lblExpiryDate" runat="server" Text='<%#Eval("ExpiryDate", "{0:dd/MM/yyyy}") %>'>
                            </asp:Label>
                        </ItemTemplate>
                    </asp:TemplateField>
                </Columns>
                <FooterStyle BackColor="#5D7B9D" Font-Bold="True" ForeColor="White" />
                <RowStyle BackColor="#F7F6F3" ForeColor="#333333" />
                <EditRowStyle BackColor="#FFFFC0" />
                <SelectedRowStyle BackColor="#E2DED6" Font-Bold="True" ForeColor="#333333" />
                <PagerStyle BackColor="#284775" ForeColor="White" HorizontalAlign="Center" />
                <HeaderStyle BackColor="Silver" Font-Bold="True" ForeColor="MidnightBlue" />
                <AlternatingRowStyle BackColor="White" />
            </asp:GridView>

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samtran0331Commented:
can you post your gridview?
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QPRAuthor Commented:
Below, let me know if you need the rest of the page code.
As I said working fine up until a particular day and then flop!

 <asp:GridView ID="GridView1" runat="server" DataSourceID="SqlDataSource3" CellPadding="4" ForeColor="#333333" GridLines="None" Width="672px" AutoGenerateColumns="False" DataKeyNames="NoticeID">
        <Columns>
            <asp:CommandField ShowDeleteButton="True" ShowEditButton="True" />
            <asp:BoundField DataField="NoticeID" HeaderText="NoticeID" InsertVisible="False" ReadOnly="True" SortExpression="NoticeID" Visible="False" />
            <asp:BoundField DataField="NoticeText" HeaderText="Notice" SortExpression="NoticeText" />
            <asp:TemplateField HeaderText="StartDate">
                    <EditItemTemplate>
                        <asp:TextBox ID="txtStartDate" runat="server" Text='<%#Bind("StartDate", "{0:dd/MM/yyyy}") %>' />
                    </EditItemTemplate>
                    <ItemTemplate>
                        <asp:Label ID="lblStartDate" runat="server" Text='<%#Eval("StartDate", "{0:dd/MM/yyyy}") %>' />
                    </ItemTemplate>
           </asp:TemplateField>
           <asp:TemplateField HeaderText="StartDate">
                    <EditItemTemplate>
                        <asp:TextBox ID="txtStartDate" runat="server" Text='<%#Bind("ExpiryDate", "{0:dd/MM/yyyy}") %>' />
                    </EditItemTemplate>
                    <ItemTemplate>
                        <asp:Label ID="lblStartDate" runat="server" Text='<%#Eval("ExpiryDate", "{0:dd/MM/yyyy}") %>' />
                    </ItemTemplate>
           </asp:TemplateField>
            <asp:BoundField DataField="ExpiryDate" HeaderText="Expiry Date" SortExpression="ExpiryDate" />
        </Columns>
        <FooterStyle BackColor="#5D7B9D" ForeColor="White" Font-Bold="True"></FooterStyle>
        <RowStyle BackColor="#F7F6F3" ForeColor="#333333"></RowStyle>
            <EditRowStyle BackColor="#FFFFC0"></EditRowStyle>
            <SelectedRowStyle BackColor="#E2DED6" ForeColor="#333333" Font-Bold="True"></SelectedRowStyle>
            <PagerStyle BackColor="#284775" ForeColor="White" HorizontalAlign="Center"></PagerStyle>
            <HeaderStyle BackColor="Silver" ForeColor="MidnightBlue" Font-Bold="True"></HeaderStyle>
            <AlternatingRowStyle BackColor="White"></AlternatingRowStyle>
    </asp:GridView>
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QPRAuthor Commented:
well spotted thanks!
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