Exception Info: System.Net.Sockets.SocketException
  at System.Net.Sockets.NetworkStream.Read(Byte[], Int32, Int32)
Inner Exception:
Exception Info: System.IO.IOException
  at System.Net.ConnectStream.Read(Byte[], Int32, Int32)
  at System.IO.Stream.InternalCopyTo(System.IO.Stream, Int32)
  at Telerik.Windows.Documents.Layout.ExtensionMethods.ToMemoryStream(System.IO.Stream)
  at Telerik.Windows.Documents.Model.ImageInline.<SetStreamFromUriSource>b__0(System.Object, StreamReadyEventArgs)
  at Telerik.Windows.Documents.Utils.ResourceStreamLocator.OnStreamReady()
  at System.Net.WebClient.OnOpenReadCompleted(System.Net.OpenReadCompletedEventArgs)
  at System.Windows.Threading.ExceptionWrapper.InternalRealCall(System.Delegate, System.Object, Int32)
  at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(System.Object, System.Delegate, System.Object, Int32, System.Delegate)
  at System.Windows.Threading.DispatcherOperation.InvokeImpl()
  at MS.Internal.CulturePreservingExecutionContext.CallbackWrapper(System.Object)
  at System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
  at System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object, Boolean)
  at System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object)
  at MS.Internal.CulturePreservingExecutionContext.Run(MS.Internal.CulturePreservingExecutionContext, System.Threading.ContextCallback, System.Object)
  at System.Windows.Threading.DispatcherOperation.Invoke()
  at System.Windows.Threading.Dispatcher.ProcessQueue()
  at System.Windows.Threading.Dispatcher.WndProcHook(IntPtr, Int32, IntPtr, IntPtr, Boolean ByRef)
  at MS.Win32.HwndWrapper.WndProc(IntPtr, Int32, IntPtr, IntPtr, Boolean ByRef)
  at MS.Win32.HwndSubclass.DispatcherCallbackOperation(System.Object)
  at System.Windows.Threading.ExceptionWrapper.InternalRealCall(System.Delegate, System.Object, Int32)
  at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(System.Object, System.Delegate, System.Object, Int32, System.Delegate)
  at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(System.Windows.Threading.DispatcherPriority, System.TimeSpan, System.Delegate, System.Object, Int32)
  at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr, Int32, IntPtr, IntPtr)
  at MS.Win32.UnsafeNativeMethods.DispatchMessage(System.Windows.Interop.MSG ByRef)
  at System.Windows.Threading.Dispatcher.PushFrameImpl(System.Windows.Threading.DispatcherFrame)
  at System.Windows.Application.RunDispatcher(System.Object)
  at System.Windows.Application.RunInternal(System.Windows.Window)

  public class MyRenderer : SpreadStreamExportRenderer
  {
      public override void SetCellValue(DataType dataType, object value)
      {
          DateTime date;
          if (value != null && DateTime.TryParse(value.ToString(), out date))
          {
              base.SetCellValue(DataType.DateTime, date);
              var cell = this.GetCell() as ICellExporter;
              cell.SetFormat(new SpreadCellFormat() { NumberFormat = "yyyy-MM-dd HH:mm:ss" });
              return;
          }
          base.SetCellValue(dataType, value);
      }
  }

 spreadStreamExport.RunExportAsync(FILENAME, new MyRenderer(), options);

<Application x:Class="WpfApp1.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:WpfApp1" 
             xmlns:telerikDialogs="clr-namespace:Telerik.Windows.Controls.RichTextBoxUI.Dialogs;assembly=Telerik.Windows.Controls.RichTextBox"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="/Telerik.Windows.Themes.Fluent;component/Themes/System.Windows.xaml"/>
                <ResourceDictionary Source="/Telerik.Windows.Themes.Fluent;component/Themes/Telerik.Windows.Controls.xaml"/>
                <ResourceDictionary Source="/Telerik.Windows.Themes.Fluent;component/Themes/Telerik.Windows.Controls.ImageEditor.xaml"/>
                <ResourceDictionary Source="/Telerik.Windows.Themes.Fluent;component/Themes/Telerik.Windows.Controls.Navigation.xaml"/>
                <ResourceDictionary Source="/Telerik.Windows.Themes.Fluent;component/Themes/Telerik.Windows.Controls.Input.xaml"/>
                <ResourceDictionary Source="/Telerik.Windows.Themes.Fluent;component/Themes/Telerik.Windows.Controls.RibbonView.xaml"/>
                <ResourceDictionary Source="/Telerik.Windows.Themes.Fluent;component/Themes/Telerik.Windows.Controls.RichTextBox.xaml"/>
            </ResourceDictionary.MergedDictionaries>            
            <Style TargetType="telerikDialogs:LineNumberingDialog" BasedOn="{StaticResource RadWindowStyle}"/>
            <Style TargetType="telerikDialogs:LineNumberingDistanceNumericUpDown" BasedOn="{StaticResource RadNumericUpDownStyle}"/>
        </ResourceDictionary>
    </Application.Resources>
</Application>