StorageFolder.CreateFileAsync crashes when called from App.OnSuspending

My Win RT application which has worked with VS2012RC on a Windows 8 beta, has now with the final versions of visual studio and windows 8 pro the problem, that creating/opening a file within OnSuspending only works if I set a debugger-breakpoint to the file creation method.

private void OnSuspending(object sender, SuspendingEventArgs e){                        
     var deferral = e.SuspendingOperation.GetDeferral();                       
     if (null != m_document) Save();
     deferral.Complete();
}

async void Save(){
    var folder = KnownFolders.DocumentsLibrary;       
    var file = await folder.CreateFileAsync(GetFileName(),Windows.Storage.CreationCollisionOption.ReplaceExisting);                

    var xDoc = GetXDocument();
    using (var stream = await file.OpenStreamForWriteAsync()){
       xDoc.Save(stream);                    
    }           
}

• If I set a breakpoint on StorageFile file = await folder.CreateFileAsync(..., the debugger enters the and if I continue, all works fine.

• However if I dont set a breakpoint, the file will be created, but the content of the xml will not be saved (the file rests empty).

• If I set a breakpoint below of the line StorageFile file = await folder.CreateFileAsync(..., the debugger never enters!

Has anyone an idea? I have also tested a version which uses folder.OpenStreamForWriteAsync, with the very same effect.