Validation ControlTemplate string always appearing even when no Validation Error

Question

I have created a style in order to specify the validation ControlTemplate to use when there is a validation error in some of my textboxes. The validation errors get caught, as I can see my style being used (the default red border for the textbox, and my added light pink background with a red "!!!" string), but the problem is the "!!!" red string is always there, even though there are no validation errors (though the pink background and red border disappear). I am using IDataErrorInfo in order to validation the textboxes.

Here is my xaml style code:

<Style x:Key="ErrorValidationTextBox" TargetType="{x:Type pres:OneTextBox}">
    <Style.Triggers>
        <Trigger Property="Validation.HasError" Value="True">
            <Setter Property="Background" Value="LightPink"></Setter>
        </Trigger>
    </Style.Triggers>
    <Setter Property="Validation.ErrorTemplate">
        <Setter.Value>
            <ControlTemplate>
                <DockPanel LastChildFill="True">
                    <AdornedElementPlaceholder x:Name="ControlWithError"/>
                    <TextBlock DockPanel.Dock="Right"
                                Foreground="Red"
                                FontSize="12pt"
                                FontWeight="Bold"
                                Margin="-18,0,0,0"
                                Text="!!!">
                    </TextBlock>
                </DockPanel>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

I tried adding this line to the < TextBlock > node but it didn't work:

Visibility="{Binding Validation.HasError, Source={RelativeSource Self}, Converter={StaticResource BoolToHiddenOrVisibleConverter}}"

My question is: How can I make the "!!!" red string only appear when the Validation.HasError flag is set to true?

Here is how I declared one of my textboxes, as a reference. The OneTextBox is a control that encapsulates the regular WPF TextBox and adds some functionalities (as I am using a custom framework):

    <pres:OneTextBox Grid.Row="0" Watermark="Name..." Margin="85,12,0,0" Style="{StaticResource ErrorValidationTextBox}"
                     Text="{Binding Path=InterfaceSpecification.Name, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True, NotifyOnValidationError=True}"
                     AcceptsReturn="False" MaxLines="1" Height="22" VerticalAlignment="Top"
                     HorizontalAlignment="Left" Width="300" />

EDIT: I am getting the following binding error when running in debug:

BindingExpression path error: 'Validation' property not found on 'object' ''RelativeSource' (HashCode=58276509)'. BindingExpression:Path=Validation.HasError; DataItem='RelativeSource' (HashCode=58276509); target element is 'TextBlock' (Name=''); target property is 'Visibility' (type 'Visibility')

EDIT2: Here is how I implement the IDataErrorInfo in my class:

    public string Error
    {
        get { return mError; }
        set { mError = value; }
    }

    public string this[string columnName]
    {
        get
        {
            switch (columnName)
            {
                case "Name":
                    if (string.IsNullOrWhiteSpace(Name))
                    {
                        Error = "The name cannot be null, empty or contain only white spaces";
                    }
                    else if (Name.StartsWith(" "))
                    {
                        Error = "The name cannot start with a white spaces";
                    }
                    else if (Name.IndexOfAny(Path.GetInvalidPathChars()) != -1)
                    {
                        Error = "The name cannot contain invalid characters";
                    }
                    else
                    {
                        Error = null;
                    }
                    break;
            }

            return Error;
        }
    }

Answer 1

This is surely working

<Style x:Key="ErrorValidationTextBox" TargetType="{x:Type pres:OneTextBox}">
<Style.Triggers>
    <Trigger Property="Validation.HasError" Value="True">
        <Setter Property="Background" Value="LightPink"></Setter>
    </Trigger>
</Style.Triggers>
<Setter Property="Validation.ErrorTemplate">
    <Setter.Value>
        <ControlTemplate>
            <DockPanel LastChildFill="True">
                <AdornedElementPlaceholder x:Name="ControlWithError"/>
                <TextBlock DockPanel.Dock="Right"
                            Foreground="Red"
                            FontSize="12pt"
                            FontWeight="Bold"
                            Margin="-18,0,0,0"
                            Text="!!!"
                            Visibility="{Binding ElementName=ControlWithError, Path=AdornedElement.(Validation.HasError),Converter={StaticResource BoolToHiddenOrVisibleConverter}}"   >
                </TextBlock>
            </DockPanel>
        </ControlTemplate>
    </Setter.Value>
</Setter>