Button content local resource disappears after click

Hi,

the "PluralSight Xaml Patterns - Composition Pattern" course shows a nice way to add assets to controls.
I downloaded a bunch of assets from Xamalot at wrapped them as advised into a resource dictionary and viewbox:

<ResourceDictionary ...<Viewbox x:Key="Xamalot.Add"><Canvas Width="35.0013875961304" Height="35.0001373291016">

Next I merged those resource dictionaries to my project and added the assets as local resources to the content of my buttons.

Starting my application the assets are shown as excepted, but if I open a other window by clicking on on of those buttons, its asset disappears!

Here is how I defined the button in XAML:
<Button Content="{DynamicResource Xamalot.Add}" MinWidth="60" Click="OnCreateNewRecipe"/>

What is it I am doing wrong?

Regards
Rainer