Replace obsolete Animation.RepeatCount property with IterationCount
MODIFY
WalletWasabi.Gui/Controls/Spinner.xaml
WalletWasabi.Gui/Controls/Spinner.xaml
<Grid.Styles>
<Style Selector="DrawingPresenter">
<Style.Animations>
- <Animation Duration="0:0:2.5" RepeatCount="Loop">
+ <Animation Duration="0:0:2.5" IterationCount="Infinite">
<KeyFrame Cue="100%">
<Setter Property="RotateTransform.Angle" Value="360" />
</KeyFrame>

Replace obsolete Animation.RepeatCount property with IterationCount