Commented Issue: XAMLParseException with BusyIndicator [4958]

I'm using november release of the toolkit, in my project I get a XAMLParseException(Line:0, Position:0) in Blend when I'm using the BusyIndicator.
But when I take the BusyIndicator out of the Page, everything works fine.

I've read that the cause of the XAMLParseException is a wrong TargetType in a Style File, but I couldn't find any Reference of the BusyIndicator in such files.

Additionally I have to mention that to BusyIndicator is the first Element in the MainPage.xaml and it wraps all the content and I'm using the Q3 2009 Telerik Controls for Silverlight.

<UserControl 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"
mc:Ignorable="d"
xmlns:controlsToolkit="clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls.Toolkit"
x:Class="XXX"
d:DesignWidth="1280"
d:DesignHeight="922"
Language="de-AT">
<controlsToolkit:BusyIndicator x:Name="activityControl" IsBusy="False" BusyContent="Loading">
<Grid>
<!-- the controls on the page-->
</Grid>
</controlsToolkit:BusyIndicator>
Comments: ** Comment from web user: xusan **

Same error in Blend when tring to use any control from Toolkit, I'm using MS Blend for Silverlight 5 and Toolkit December 2011, really tired of seeing that exception, is there any workaround ?