Problems with Microsoft Expression Blend 4 SketchFlow

Question

I got a serious problem with MS Expression Blend 4 (MSEXB4): I made a prototype for an application. When I start the project by hitting F5 MSEXB4 crashes.

When I remove the sketchflow-map it works, but no screen is loaded. But I want to have a sketchflowmap to show some screens -.-

I started debugging in VS2010:

The error:

{"Unable to generate a temporary class (result=1).\r\nerror CS0009: Metadata file 'c:\\Windows\\Microsoft.NET\\assembly\\GAC_MSIL\\WindowsBase\\v4.0_4.0.0.0__31bf3856ad364e35\\WindowsBase.dll' could not be opened -- 'Es wurde versucht, eine Datei mit einem falschen Format zu laden. '\r\n"}

The German part at the end means "Tried to load a file with a wrong format".

System.InvalidOperationException was unhandled
  Message=Unable to generate a temporary class (result=1).
error CS0009: Metadata file 'c:\Windows\Microsoft.NET\assembly\GAC_MSIL\WindowsBase\v4.0_4.0.0.0__31bf3856ad364e35\WindowsBase.dll' could not be opened -- 'Es wurde versucht, eine Datei mit einem falschen Format zu laden. '

  Source=System.Xml
  StackTrace:
       at System.Xml.Serialization.Compiler.Compile(Assembly parent, String ns, XmlSerializerCompilerParameters xmlParameters, Evidence evidence)
       at System.Xml.Serialization.TempAssembly.GenerateAssembly(XmlMapping[] xmlMappings, Type[] types, String defaultNamespace, Evidence evidence, XmlSerializerCompilerParameters parameters, Assembly assembly, Hashtable assemblies)
       at System.Xml.Serialization.XmlSerializer.GenerateTempAssembly(XmlMapping xmlMapping, Type type, String defaultNamespace)
       at System.Xml.Serialization.XmlSerializer..ctor(Type type, String defaultNamespace)
       at Microsoft.Expression.Prototyping.Data.Serializer.Serialize(Data data, Stream stream)
       at Microsoft.Expression.Prototyping.DataModel.Model.Serialize(Stream stream)
       at Microsoft.Expression.Prototyping.HostEnvironment.FlowgraphDocument.SaveCore(Stream stream)
       at Microsoft.Expression.Framework.Documents.UndoDocument.Save()
       at Microsoft.Expression.Framework.Documents.DocumentUtilities.SaveDocument(IDocument document, Boolean saveAsOnFailure, Boolean forceSave, IMessageDisplayService messageManager)
       at Microsoft.Expression.Project.ProjectItem.SaveDocumentFile()
       at Microsoft.Expression.Project.SolutionBase.<>c__DisplayClass27.<SaveProjectItems>b__25(IProjectItem item)
       at System.Linq.Enumerable.Any[TSource](IEnumerable`1 source, Func`2 predicate)
       at Microsoft.Expression.Project.SolutionBase.<>c__DisplayClass27.<SaveProjectItems>b__22()
       at Microsoft.Expression.Project.ErrorHandling.HandleBasicExceptions(Action action, Action`1 handledExceptionAction)
       at Microsoft.Expression.Project.SolutionBase.SaveProjectItems(Boolean promptBeforeSaving, Boolean saveActiveDocument)
       at Microsoft.Expression.Project.SolutionBase.Save(Boolean promptBeforeSaving)
       at Microsoft.Expression.Project.Commands.ProjectCommandExtensions.SaveSolution(IProjectCommand source, Boolean promptBeforeSaving)
       at Microsoft.Expression.Project.Commands.TestProjectCommand.BuildAndRun(IProjectBuildContext buildTarget, IExecutable executable)
       at Microsoft.Expression.Project.Commands.TestProjectCommand.<>c__DisplayClass2.<Execute>b__0()
       at Microsoft.Expression.Project.ErrorHandling.HandleBasicExceptions(Action action, Action`1 handledExceptionAction)
       at Microsoft.Expression.Project.ServiceExtensions.ErrorHandling.ErrorHandlingServiceExtensions.ExceptionHandler(IServiceProvider source, Action action, Func`1 errorMessage)
       at Microsoft.Expression.Project.Commands.ProjectCommandExtensions.HandleBasicExceptions(IProjectCommand source, Action action)
       at Microsoft.Expression.Project.Commands.TestProjectCommand.Execute()
       at Microsoft.Expression.Framework.Commands.CommandTarget.Execute(String commandName, CommandInvocationSource invocationSource)
       at Microsoft.Expression.Framework.Commands.CommandService.Execute(String commandName, CommandInvocationSource invocationSource)
       at Microsoft.Expression.Framework.UserInterface.CommandBarButton.<Me_Click>b__0()
       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)
       at System.Windows.Threading.DispatcherOperation.InvokeImpl()
       at System.Windows.Threading.DispatcherOperation.InvokeInSecurityContext(Object state)
       at System.Threading.ExecutionContext.runTryCode(Object userData)
       at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData)
       at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Windows.Threading.DispatcherOperation.Invoke()
       at System.Windows.Threading.Dispatcher.ProcessQueue()
       at System.Windows.Threading.Dispatcher.WndProcHook(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)
       at System.Windows.Threading.Dispatcher.InvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
       at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
       at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
       at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
       at System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
       at System.Windows.Threading.Dispatcher.Run()
       at System.Windows.Application.RunDispatcher(Object ignore)
       at System.Windows.Application.RunInternal(Window window)
       at System.Windows.Application.Run(Window window)
       at System.Windows.Application.Run()
       at Microsoft.Expression.Framework.ExpressionApplication.RunApplication()
       at Microsoft.Expression.Application.Main(String[] args)
  InnerException: 

I already googled around a little bit, but couldn't find something.

Thanks! tobi