Basing it on the link you posted,
The class RoutedEventTrigger
expects a RoutedEvent
and your xaml is not able to convert the string Validation.Error
to the required type.
so switch
<i:Interaction.Triggers>
<view_model:RoutedEventTrigger RoutedEvent="Validation.Error">
<mvvm:EventToCommand Command="{Binding MyCmd}" PassEventArgsToCommand="True" />
</view_model:RoutedEventTrigger>
</i:Interaction.Triggers>
to
<i:Interaction.Triggers>
<view_model:RoutedEventTrigger RoutedEvent="{x:Static Validation.ErrorEvent}">
<mvvm:EventToCommand Command="{Binding MyCmd}" PassEventArgsToCommand="True" />
</view_model:RoutedEventTrigger>
</i:Interaction.Triggers>
and it should be fine