Blurry textblock text with the same code

Question

I have 2 different applications. Both applications check the connection to a WCF-Service. The result is bound to the view.

The XAML code of for both TextBlocks in the first and second application are exactly the same. Nevertheless, one of the textblocks got a strange blur effect. The right one on the picture:

XAML code for both textblocks:

   <!--Connection Status-->
    <StackPanel Orientation="Horizontal" VerticalAlignment="Top" HorizontalAlignment="Center" Grid.Row="2" Grid.Column="1">
        <TextBlock Text="Server: " Foreground="White" FontSize="20" FontFamily="Calibri" >
            <TextBlock.Effect>
                <DropShadowEffect ShadowDepth="4" Direction="330" Opacity="0.5" BlurRadius="4" Color="Black"/>
            </TextBlock.Effect>
        </TextBlock>
        <TextBlock Text="{Binding connectionStatus}" Foreground="{Binding connectionColor}" FontSize="20" FontFamily="Calibri" >
            <TextBlock.Effect>
                <DropShadowEffect ShadowDepth="4" Direction="330" Color="Black" Opacity="0.5" BlurRadius="4" />
            </TextBlock.Effect>
        </TextBlock>
    </StackPanel>

What could cause this strange behavior?

Edit: The possible duplicate WPF Blurry fonts issue- Solutions does not address my problem. I could not find a solution or an explanation for this strange behavior. I don't have a general problem with WPF Text rendering.