The issue that we know arises when you dock a form/usercontrol using the DockControl method and then save and try to load the RadDock layout. If you want to workaround this exception, you should use the API which I demonstrated above. When the forms/controls are docked through this API, there are no issues on my side when I save and load the RadDock layout.
If your scenario is different, please open a new support ticket and send me a sample project. This will allow me to help you further.
the Telerik team