Value cannot be null error in System.Security.SecurityElement FromString

I have a WPF application and some of my customers are reporting this issue that occurs at launch. It doesn't occurs very often and I am unable to reproduce it. The data I have from some customers is showing that it happens on various type of computer (64 and 32 bits, intel, amd, pentium, Windows 7 or 8, etc.), I haven't found any correlation between the systems yet.

The only details I have at this time are these ones :

EXCEPTION MESSAGE : Value cannot be null. Parameter name: xml

TARGET SITE : System.Security.SecurityElement FromString(System.String)

INNER EXCEPTION :

SOURCE : mscorlib

STACKTRACE : at System.Security.SecurityElement.FromString(String xml) at System.Security.BuiltInPermissionSets.GetOrDeserializePermissionSet(NamedPermissionSet& permissionSet, String permissionSetXml) at System.Security.Policy.PolicyLevel.GetBuiltInSet(String name) at System.Security.Permissions.PermissionSetAttribute.CreatePermissionSet() at System.Security.PermissionSet.CreateSerialized(Object[] attrs, Boolean serialize, Byte[]& nonCasBlob, PermissionSet& casPset, HostProtectionResource fullTrustOnlyResources, Boolean allowEmptyPermissionSets) at DevExpress.Xpf.Docking.DockLayoutManager.UnsubscribeSystemEvents() at DevExpress.Xpf.Docking.DockLayoutManager.OnLoaded() at DevExpress.Xpf.Docking.VisualElements.psvControl.psvControl_Loaded(Object sender, RoutedEventArgs e) 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.RaiseEvent(RoutedEventArgs e) at System.Windows.BroadcastEventHelper.BroadcastEvent(DependencyObject root, RoutedEvent routedEvent) at System.Windows.BroadcastEventHelper.BroadcastLoadedEvent(Object root) at MS.Internal.LoadedOrUnloadedOperation.DoWork() at System.Windows.Media.MediaContext.FireLoadedPendingCallbacks() at System.Windows.Media.MediaContext.FireInvokeOnRenderCallbacks() at System.Windows.Media.MediaContext.RenderMessageHandlerCore(Object resizedCompositionTarget) at System.Windows.Media.MediaContext.RenderMessageHandler(Object resizedCompositionTarget) at System.Windows.Media.MediaContext.Resize(ICompositionTarget resizedCompositionTarget) at System.Windows.Interop.HwndTarget.OnResize() at System.Windows.Interop.HwndTarget.HandleMessage(WindowMessage msg, IntPtr wparam, IntPtr lparam) at System.Windows.Interop.HwndSource.HwndTargetFilterMessage(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)

I really dont know where to dig so if someone have an idea where I should begin, that would be much appreciated.

Thanks