WPF ListView & Converter - wpf

Question

I have a ListView with a GridViewColumn that has an image in it, it uses a converter to convert a bool to a ImageSource. It was working perfectly for months and now it suddenly just shows a red dot instead of my image.
Converter:
public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
if ((bool)value)
return new BitmapImage(new Uri(#"/Dionysus.Styling;component/Images/Actions-dialog-ok-apply-icon.png", UriKind.RelativeOrAbsolute));
else return
null;
}
Xaml:
<GridViewColumn Header="">
<GridViewColumn.CellTemplate>
<DataTemplate>
<Image Source="{Binding Path=IsDelayedOrPreferred, Converter={StaticResource DelayConverter}, Mode=TwoWay}" />
</DataTemplate>
</GridViewColumn.CellTemplate>
</GridViewColumn>
Result:
Has anyone seen this behaviour before or know what might have changed. The only recent changes that I made was upgrading to VS 2013 but all other ListViews with Converters
are still working as expected.
Style:
<Style TargetType="{x:Type ListView}">
<Setter Property="BorderBrush" Value="White"/>
<Setter Property="Foreground" Value="Black"/>
<Setter Property="AlternationCount" Value="2"/>
<Setter Property="Background" Value="WhiteSmoke"/>
<EventSetter Event="Loaded" Handler="ListView_Loaded"/>
</Style>
<Style TargetType="{x:Type GridViewColumnHeader}">
<Setter Property="HorizontalContentAlignment" Value="Left"/>
<Setter Property="Margin" Value="1,0"/>
</Style>
<Style TargetType="{x:Type ListViewItem}" x:Key="ListViewStyle">
<Style.Triggers>
<Trigger Property="ItemsControl.AlternationIndex" Value="0">
<Setter Property="Background" Value="WhiteSmoke"></Setter>
</Trigger>
<Trigger Property="ItemsControl.AlternationIndex" Value="1">
<Setter Property="Background" Value="White"></Setter>
</Trigger>
<DataTrigger Binding="{Binding IsFirst}" Value="True">
<Setter Property="Background" Value="LightGreen"></Setter>
</DataTrigger>
<DataTrigger Binding="{Binding IsLastUnpaid}" Value="True">
<Setter Property="Background" Value="LightSalmon"></Setter>
</DataTrigger>
<Trigger Property="IsSelected" Value="True">
<Setter Property="Background" Value="#FF41B1E1"></Setter>
</Trigger>
</Style.Triggers>
<Setter Property="Height" Value="20" />
<Setter Property="HorizontalAlignment" Value="Left"/>
</Style>
UPDATE:
I recently changed from a ListView to a DataGrid and then I see a little exclamation mark as if there is data validation errors on the DataGrid.
Any ideas?