Invalid postback or callback argument - button in Telerik grid - asp.net

Question

Very famous error message (see below), judging by the number of Google results. But every one of them I've seen suggests to set EnableEventValidation to false. I have searched my entire codebase, and I cannot find the string "EnableEventValidation" anywhere. Moreover, this code used to work; something I have done has obviously broken the pages. But what?
The error happens when I click on a button inside a Telerik RadGrid, declared as:
<telerik:RadGrid ID="MyGrid" Width="100%" ItemStyle-BorderColor="Gainsboro"
ItemStyle-BorderStyle="Solid" ItemStyle-BorderWidth="1px" ActiveItemStyle-BackColor="Bisque"
SelectedItemStyle-BackColor="Black" AllowPaging="True" PageSize="15" runat="server"
AllowSorting="true" OnItemCommand="MyGrid_ItemCommand" AutoGenerateColumns="false"
OnNeedDataSource="MyGrid_NeedDataSource" GridLines="Horizontal" AllowMultiRowSelection="false"
Skin="Black">
<GroupingSettings CaseSensitive="false" />
<MasterTableView Width="100%" DataKeyNames="ID" AllowFilteringByColumn="false" Font-Names="Arial"
Font-Size="10px">
<Columns>
<telerik:GridButtonColumn ButtonType="PushButton" Text="Cancel" CommandName="Cancel"
ConfirmText="Are you sure you want to cancel this?">
</telerik:GridButtonColumn>
...
</Columns>
</MasterTableView>
<PagerStyle Mode="NextPrevAndNumeric" />
<FilterMenu EnableTheming="True">
<CollapseAnimation Duration="200" Type="OutQuint" />
</FilterMenu>
</telerik:RadGrid>
click on the "Cancel" button, and here's the famous error:
Invalid postback or callback argument. Event validation is enabled using <pages enableEventValidation="true"/> in configuration or <%# Page EnableEventValidation="true" %> in a page. For security purposes, this feature verifies that arguments to postback or callback events originate from the server control that originally rendered them. If the data is valid and expected, use the ClientScriptManager.RegisterForEventValidation method in order to register the postback or callback data for validation.

Answer 1

Here's the problem: in my Page_Load method I had:
protected void Page_Load(object sender, EventArgs e) {
MyGrid.Rebind();
}
The rebinding of the grid on postback was obviously screwing something up. I changed it to:
protected void Page_Load(object sender, EventArgs e) {
if (!IsPostBack) {
MyGrid.Rebind();
}
}
and everything is working now.

Answer 2

I had the same problem but I had no Grid.Rebind() or Grid.Databind() in my NeedDataSource method or Page_Load method. This happened just after I drag a column to be grouped and then order the grouped column ASC/DESC
I simply added
EnableEventValidation="false"
in the <%# Page %> tag of my .aspx page. The ordering fails but at least I no longer get the error. As a note everything else works perfectly except the ordering of a grouped column
here is the code I use in the NeedDataSource method
protected void RadGrid1_NeedDataSource(object sender, GridNeedDataSourceEventArgs e)
{
String connstr = ConfigurationManager.ConnectionStrings["PrimeIntegartionsConnectionString"].ConnectionString;
SqlDataSource Ds = new SqlDataSource(connstr, BuildSql()); //buildsql simply returns a SQLSelect String "select * from example"
RadGrid1.DataSource = Ds;
}