Using a StaticResource SolidColorBrush to define the Gradient Stop Colors

Ok, I found the problem:

Using Color and not SolidColorBrush..

<Color x:Key="colorbrushMedium">#FF5A5A5A</Color>
<Color x:Key="colorbrushDark">#FF222222</Color>
<LinearGradientBrush>
    <GradientStop Color="{StaticResource colorbrushMedium}"/>
    <GradientStop Color="{StaticResource colorbrushDark}" Offset="1"/>
</LinearGradientBrush>

This seems to solve my problem!