To reproduce:
- Add a collapsible panel to ToolWindow.
- Collapse the panel and redock the tool window.
- The panel cannot be expanded any more.

Workaround:
List<Rectangle> list;
void radDock1_DockStateChanging(object sender, Telerik.WinControls.UI.Docking.DockStateChangingEventArgs e)
{
    list = new List<Rectangle>();
    foreach (var item in e.NewWindow.Controls)
    {
        if (item is RadCollapsiblePanel)
        {
            list.Add(((RadCollapsiblePanel)item).OwnerBoundsCache);
        }
    }
}

void radDock1_DockStateChanged(object sender, Telerik.WinControls.UI.Docking.DockWindowEventArgs e)
{
    int count = 0;
    foreach (var item in e.DockWindow.Controls)
    {
        if (item is RadCollapsiblePanel)
        {
            ((RadCollapsiblePanel)item).OwnerBoundsCache = list[count++];
        }
    }
}