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>