Q:

Converter cannot be applied to a property that expects the type IValueConverter

I have a converter which implements IValueConverter but is not bindable to a property.
public class StatusToBrushConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return Brushes.Red;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

}
Within XAML I added the converter as resource and added Binding to the Element
<UserControl.Resources>
    <Converters:StatusToBrushConverter x:Key="StatusConverter"/>
</UserControl.Resources>            

<Rectangle Fill="{Binding Status, Converter={StaticResource StatusConverter}, ElementName=userControl}"/>

But I constantly get the error

An object of the type "StatusToBrushConverter" cannot be applied to a property that expects the type "System.Windows.Data.IValueConverter"

But the Converter implements the Interface IValueConverter. I tried several things:

Rebuild, Clean, Build, Build Solution, etc.
Completely new Converter --> Same

Previously written converters work. Thoughts?

A:

Make sure that your StatusToBrushConverter class really implements the correct IValueConverter interface by using its fully qualified named:
public class StatusToBrushConverter : System.Windows.Data.IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return Brushes.Red;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

You may also try to temporarily use the property element syntax for debugging purposes:
<Rectangle>
    <Rectangle.Fill>
        <Binding Path="Status" ElementName="userControl">
            <Binding.Converter>
                <local:StatusToBrushConverter />
            </Binding.Converter>
        </Binding>
    </Rectangle.Fill>
</Rectangle>

A:

The solution was following: The Converter was within a class library. Should not be a problem and was for other converters no problem. But I moved the Converter in  the WPF Project and now it worked. I did not change anything in the Converter. 
The initial error with the error message was disappeared when I turned on my PC this morning. Several restarts of visual studio were not enough. Just the PC restart did the trick.
Thank you for your proposals. I tried all of them.

