RadGrid throws a "Failed to load viewstate" exception in a specific setup with two level hierarchy and DateTimeColumns. 

Steps to reproduce:
1. Expand an item
2. Click on "Add new record button" in the expanded detail table view
3. Click "Cancel"
4. Click on"Add new record" this time on the master table view

Code to reproduce:

<telerik:RadGrid ID="rgComments"
    runat="server"
    AllowPaging="True"
    AutoGenerateColumns="false"
    DataSourceID="dsSpell">
    <MasterTableView Width="100%"
        CommandItemDisplay="Top" DataKeyNames="OrderID">
        <DetailTables>
            <telerik:GridTableView DataKeyNames="OrderID" DataSourceID="dsComment" Width="100%"
                runat="server" CommandItemDisplay="Top">
                <ParentTableRelation>
                    <telerik:GridRelationFields DetailKeyField="OrderID" MasterKeyField="OrderID"></telerik:GridRelationFields>
                </ParentTableRelation>
                <Columns>
                    <telerik:GridDateTimeColumn DataField="OrderDate" PickerType="DateTimePicker">
                    </telerik:GridDateTimeColumn>
                </Columns>
            </telerik:GridTableView>
        </DetailTables>
        <Columns>
            <telerik:GridDateTimeColumn DataField="OrderID" />
        </Columns>
    </MasterTableView>
</telerik:RadGrid>
<br />
<asp:SqlDataSource runat="server" ID="dsComment" ConnectionString="<%$ ConnectionStrings:NorthwindConnectionString %>"
    SelectCommand="SELECT [OrderID], [OrderDate] FROM [Orders]"></asp:SqlDataSource>
<asp:SqlDataSource runat="server" ID="dsSpell" ConnectionString="<%$ ConnectionStrings:NorthwindConnectionString %>"
    SelectCommand="SELECT [OrderID], [OrderDate] FROM [Orders]"></asp:SqlDataSource>

Enable drag-and-drop between cells

Since Q3 a change in the client-side binding of the GridTemplateColumn has been introduced. The column no longer automatically populates controls in the ItemTemplate. The change is result of an optimization in code regarding the virtualization and batch editing features of the grid. The new behavior is far more optimized and ensures much faster and robust operation of the control in various cases of client-side binding.