Deploy error: base class type 'RadGrid1' but type not compatible with TelerikWeb.UI.RadGrid

[Alex]

HELP!
I added a Telerik RadGrid to my aspx page and then configured my datasource. I tested in debug mode and I get this error:

The base class includes the field 'RadGrid1', but its type (Telerik.Web.UI.RadGrid) is not compatible with the type of control (Telerik.Web.UI.RadGrid).

I can't figure what is wrong??? My web.config has telerik Version=2012.3.1205.35 added and Version=2012.3.1205.40
I tried to add the version 4.0 assembly to my GAC but I got an error. Is this the problem or is there more to this?
What can I do to fix this and get it running?

Here is my Default.aspx page

<%@ Page Title="Home Page" Language="C#" MasterPageFile="~/Site.master" AutoEventWireup="true"
    CodeBehind="Default.aspx.cs" Inherits="FilterGrid._Default" %>

<%@ Register Assembly="Telerik.Web.UI" Namespace="Telerik.Web.UI" TagPrefix="telerik" %>

<asp:Content ID="HeaderContent" runat="server" ContentPlaceHolderID="HeadContent">
</asp:Content>
<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">
    <asp:ScriptManager ID="ScriptManager1" runat="server">
    </asp:ScriptManager>

    <telerik:RadGrid ID="RadGrid1" runat="server" CellSpacing="0"
        DataSourceID="SqlDataSource" GridLines="None">
<MasterTableView AutoGenerateColumns="False" DataSourceID="SqlDataSource">
<CommandItemSettings ExportToPdfText="Export to PDF"></CommandItemSettings>

<RowIndicatorColumn Visible="True" FilterControlAltText="Filter RowIndicator column">
<HeaderStyle Width="20px"></HeaderStyle>
</RowIndicatorColumn>

<ExpandCollapseColumn Visible="True" FilterControlAltText="Filter ExpandColumn column">
<HeaderStyle Width="20px"></HeaderStyle>
</ExpandCollapseColumn>

    <Columns>
        <telerik:GridBoundColumn DataField="Client"
            FilterControlAltText="Filter Client column" HeaderText="Client"
            SortExpression="Client_Name" UniqueName="Client">
        </telerik:GridBoundColumn>
        <telerik:GridBoundColumn DataField="Emp"
            FilterControlAltText="Filter Emp column" HeaderText="Emp"
            SortExpression="Emp" UniqueName="Emp">
        </telerik:GridBoundColumn>
        <telerik:GridBoundColumn DataField="ClientNum"
            FilterControlAltText="Filter ClientNum column" HeaderText="ClientNum"
            SortExpression="ClientNum" UniqueName="ClientNum">
        </telerik:GridBoundColumn>
    </Columns>

<EditFormSettings>
<EditColumn FilterControlAltText="Filter EditCommandColumn column"></EditColumn>
</EditFormSettings>
</MasterTableView>

<FilterMenu EnableImageSprites="False"></FilterMenu>
    </telerik:RadGrid>
    <asp:SqlDataSource ID="SqlDataSource"
    ConnectionString="<%$ ConnectionStrings:Matter_SearchConnectionString %>"
    SelectCommand="SELECT [Client], [Emp], [ClientNum] FROM [Table] ORDER BY [Client]"
    runat="server" ></asp:SqlDataSource>
</asp:Content>

Thanks.

[Maria Ilieva]

Hello Alex,

Review the answer provided in the support ticket you have opened for the same issue and let us know if it helps.

Kind regards,
Maria Ilieva
the Telerik team