Hello.
The problem is following.
When I remove link between 2 nodes inherited from ShapeNode and try to make new link between these nodes I catch the following exception:
29.08.2012 13:46:29. Unhandled exception catched.

System.Security.VerificationException: Operation could destabilize the runtime.
   at A.TJ`1..ctor(Int32 A)
   at A.QJ.X(NJ A, AK B, NJ C, AK D)
   at MindFusion.Diagramming.Wpf.QuickRouter.H(DiagramLink A, QJ B, List`1 C)
   at MindFusion.Diagramming.Wpf.QuickRouter.H(DiagramLink A, QJ B)
   at MindFusion.Diagramming.Wpf.QuickRouter.RouteLink(DiagramLink link)
   at MindFusion.Diagramming.Wpf.DiagramLink.JS(Boolean A, ConnectionPoint B, ConnectionPoint C, Boolean D, DiagramItemCollection E)
   at MindFusion.Diagramming.Wpf.DiagramLink.JS(Boolean A, Boolean B, DiagramItemCollection C)
   at MindFusion.Diagramming.Wpf.DiagramLink.Route()
   at Technologist.TPDEDiagram.OnDiagram_LinkCreated(Object sender, LinkEventArgs e) in D:\Work\TPDE\tpde4\TPDEDiagram_EventHadlers.cs:line 285
   at MindFusion.Diagramming.Wpf.Diagram.OnLinkCreated(LinkEventArgs e)
   at MindFusion.Diagramming.Wpf.Diagram.RaiseCreated(DiagramItem item)
   at MindFusion.Diagramming.Wpf.InteractionState.Complete(Point point, Diagram diagram)
   at MindFusion.Diagramming.Wpf.Behaviors.BehaviorBase.OnMouseUp(Point mousePosition, MouseButton mouseButton)
   at Technologist.LinkBehavior.OnMouseUp(Point mousePosition, MouseButton mouseButton) in D:\Work\TPDE\tpde4\LinkBehavior.cs:line 336
   at MindFusion.Diagramming.Wpf.Diagram.TN(MouseButtonEventArgs A, Point B)
   at MindFusion.Diagramming.Wpf.Diagram.OnPreviewMouseUp(MouseButtonEventArgs e)
   at System.Windows.UIElement.OnPreviewMouseUpThunk(Object sender, MouseButtonEventArgs e)
   at System.Windows.Input.MouseButtonEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget)
   at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
   at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
   at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
   at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
   at System.Windows.UIElement.RaiseTrustedEvent(RoutedEventArgs args)
   at System.Windows.UIElement.RaiseEvent(RoutedEventArgs args, Boolean trusted)
   at System.Windows.Input.InputManager.ProcessStagingArea()
   at System.Windows.Input.InputManager.ProcessInput(InputEventArgs input)
   at System.Windows.Input.InputProviderSite.ReportInput(InputReport inputReport)
   at System.Windows.Interop.HwndMouseInputProvider.ReportInput(IntPtr hwnd, InputMode mode, Int32 timestamp, RawMouseActions actions, Int32 x, Int32 y, Int32 wheel)
   at System.Windows.Interop.HwndMouseInputProvider.FilterMessage(IntPtr hwnd, WindowMessage msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at System.Windows.Interop.HwndSource.InputFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)

On Windows XP this problem is absent.

I haven't time now to create the test project but I include source codes and compile your dlls under Windows 7. After that I can't reproduce the problem but your and my Diagramming.Wpf, Scripting and Graph libraries have different sizes. My libraries are less than your

This is the application that uses database in internal LAN of the corporation and rertieves some data from the WCF services in this LAN. But how can it influence when I already restored diagram from the string which was recieved from the database?

No. It is desktop application which started from the local directories and I saw the problem when I started debug version of the application from Visual Studio in my working directory with solution. The directory is under version control (for this purpose we use Git)

We have upgraded to a newer version of the obfuscation software we are using, could you check if the build from the PM page works any better under your security settings?