Since I was unable to reproduce the reported problem in my end using a simple Silverlight project with following detail:

• Create Silverlight 5 project (.NET Framework 4.0) using Intersoft ClientUI MVVM Application project template
• ClientUI 11 build 206.
• Visual Studio 2013 Update 4
• Open design view and add UXClock to UXPage1.xaml

Could you please kindly add more information regarding this?

• The Silverlight project version. Is it SL 4 or SL 5 project?
• Does this happen on Visual Studio or Blend?
• Visual Studio version or Blend version.
• To determine whether the problem was caused by ClientUI control or not, please try to create a new Silverlight application by using Silverlight Application project template provided by Microsoft then add a common Silverlight controls such as: CheckBox, or TextBlock, or TextBox
• If the problem doesn't persist after adding common Silverlight control, proceed by adding ClientUI control into the xaml page.
• If the problem persist after adding ClientUI control, please send the xaml page to be investigated further.

I am using VS 2012 update 4.
My project is a SL 5 project.
I'm using ClientUI 11, build 206.
It's happening in VS 2012. 

I have created a new form matching an existing form. Everything works fine and displays properly until I add a UXTextBox to the form. As soon as I delete the UXTextBox, everything displays again.

Below is the sample page. As soon as I add the UXTextBox anywhere on the form, I get the above error.

<Intersoft:UXPage 	xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"	xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
	xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
	xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
	xmlns:Intersoft="http://intersoft.clientui.com/schemas"
	mc:Ignorable="d"
	x:Class="MyApp.Views.Adjuster.UXPage1" 
	Title="UXPage1 Page"
	d:DesignWidth="640" d:DesignHeight="480">

	<Grid x:Name="LayoutRoot">
		<Grid.Background>
			<ImageBrush AlignmentY="Bottom" AlignmentX="Right" Stretch="None" Opacity="0.5" 
						ImageSource="/MyApp;component/Assets/Images/ProductsLarge.png">
				<ImageBrush.Transform>
					<TranslateTransform X="40" Y="40"/>
				</ImageBrush.Transform>
			</ImageBrush>
		</Grid.Background>
		<Intersoft:DockPanel Margin="10" >
			<Intersoft:StylishLabel Content="Label" Intersoft:DockPanel.Dock="Top" />
			<Intersoft:UXScrollViewer Intersoft:DockPanel.IsFillElement="True" VerticalScrollBarVisibility="Auto" HorizontalScrollBarVisibility="Auto">
				<Intersoft:UXStackPanel Orientation="Vertical">
					<Intersoft:UXToolBar>
						<Intersoft:UXToolGroup>
							<Intersoft:UXToolBarButton Content="ToolBarButton"/>
							<Intersoft:UXToolBarButton Content="ToolBarButton"/>
						</Intersoft:UXToolGroup>
					</Intersoft:UXToolBar>
					<Intersoft:ExpandableGroupBox Header="General">
						<Intersoft:UXStackPanel Orientation="Horizontal">
							<Intersoft:UXItemsControl>
								<Intersoft:FieldLabel >
								</Intersoft:FieldLabel>
							</Intersoft:UXItemsControl>
						</Intersoft:UXStackPanel>
					</Intersoft:ExpandableGroupBox>
				</Intersoft:UXStackPanel>
			</Intersoft:UXScrollViewer>

		</Intersoft:DockPanel>

	</Grid>
</Intersoft:UXPage>

 Also, I can several other UX data entry controls (e.g., UXCheckBox) without issue. If I add the System.Windows.Controls.TextBox, it gives me the error.

I created a simple Silverlight project based on your description.

Visual Studio 2012 Update 4.

Here is how the design view look like after add SL TextBox into the page.

Here is how the design view look like after add SL TextBox into the page.

As shown in the above screenshots, the desig-view problem is not reproducible in my local end. I enclosed my simple project (as attachment) for your reference. Could you please kindly re-test the problem using TestDesigner2012 project and let me know the result?