The XAML team has recently reviewed this issue and will not be addressing it as at this time the team is focusing on the bugs impacting the highest number of developers. If you have encountered this issue and it is blocking for your work please contact us through the support ticketing system with details on your setup and a reference to this item.

Workaround:
In order to resolve this issue in the Silverlight version of the control all you need to do is add a Dispatcher for the SaveLayout call in the LayoutChangeEnded event.

Example:
private void OnLayoutChangeEnded(object sender, EventArgs e)
 {
     this.Dispatcher.BeginInvoke(new Action(delegate()
     {
          ...
          this.radDocking.SaveLayout(...
          ...
     }));
}