To reproduce:

Create a RadForm and use the following code:

public Form1()
{
    InitializeComponent();

    var theme = new Telerik.WinControls.Themes.Windows7Theme();
    ThemeResolutionService.ApplicationThemeName = theme.ThemeName;// "Windows7";
    this.AllowTheming = false;
}

Start the application on Windows 7, you will see that the close/minimize/maximize buttons cannot be clicked.

Workaround:

Do not set the AllowTheming property or set it to true