When I set RadWindow property VisibleTitlebar="false" then top border is missing on window.
Here is sample code:
<telerik:RadWindow runat="server" ID="GenerateReportWaitWindow" Width="440px" Height="200px" 
                    VisibleTitlebar="false" Modal="true" ReloadOnShow="false" VisibleStatusbar="false" 
                    VisibleOnPageLoad="false" EnableShadow="false" Animation="Fade" Behaviors="None"
                    Title="Čakajte prosím...">
                    <ContentTemplate>
                        <div style="min-height: 66%;">
                            <div style="padding-top: 30px; padding-bottom: 45px; text-align: center; font-weight: bold;">
                                <asp:Label ID="Label4" runat="server">Prebieha vytváranie tlačovej zostavy. Čakajte prosím.</asp:Label>
                              
                                <asp:Label ID="Label5" runat="server">(V závislosti od počtu záznamov to môže trvať aj niekoľko minút...)</asp:Label>
                                <asp:Label ID="Label6" runat="server" ForeColor="Red" Style="display: none;"></asp:Label>
                          
                            </div>
                            <div style="margin-left: auto; margin-right: auto; text-align: center;">
                                <asp:Image ID="Image2" runat="server" ImageUrl="~/Images/Animated/loading1.gif" />
                            </div>
                        </div>
                    </ContentTemplate>
                </telerik:RadWindow>

Admin comment:
Actually there is 1px top border, but it is hard to be noticed. As a temporary solution, one can increase it via the following padding:

Copy Code
<style>
    .RadWindow {
        padding: 4px 5px 5px !important;
    }
</style>