To reproduce add RadGroupBoxElement in another control (for example in a ribbon group) and use the VisualStudio2012 theme.

Workaround: either open the theme and set the GroupBoxElement theming to be reused in other controls or use the following snippet to do the same:
 private void FixTheme()
        {
            string controlType = "Telerik.WinControls.UI.RadGroupBox";
            StyleGroup styleGroup = ThemeResolutionService.GetTheme("VisualStudio2012Light").FindStyleGroup(controlType);
            styleGroup.Registrations.Add(new StyleRegistration("ElementTypeDefault", "Telerik.WinControls.UI.RadGroupBoxElement", controlType, string.Empty, string.Empty));
        }