To reproduce:
private void RadForm3_Shown(object sender, EventArgs e)
{
    Telerik.WinControls.UI.RadWaitingBar waitbar = new Telerik.WinControls.UI.RadWaitingBar();
    pnlCenter.Enabled = false;
    waitbar.AssociatedControl = pnlCenter;
    waitbar.StartWaiting();
}

Workaround:
Do not disable the control while using the waiting bar