New Post: CheckComboBox in VS 2013 Package: Could not load file or assembly

I am developing a Visual Studio 2013 Package.

I've used NuGet to get the Extended WPF Toolkit and added a CheckComboBox to one of my UserControl.

When I debug the VS Experimental Hive, the user control cannot load because an exception is shown in the XAML editor, right before the CheckComboBox declaration.

The declaration:

<UserControl
    x:Class="EditorControl"
    xmlns:xctk="http://schemas.xceed.com/wpf/xaml/toolkit"
    ...
    />

<xctk:CheckComboBox
    Delimiter=";"
    ItemsSource="{Binding ApplicabilityValues}"
    SelectedValue="{Binding Applicability}"
    SelectedItemsOverride="{Binding SelectedItems}"
    />

The exception:

XamlParseException

A first chance exception of type 'System.Windows.Markup.XamlParseException' occurred in PresentationFramework.dll

Additional information: Could not load file or assembly 'Xceed.Wpf.Toolkit, PublicKeyToken=3e4669d2f30244f4' or one of its dependencies. The parameter is incorrect. (Exception from HRESULT: 0x80070057 (E_INVALIDARG))

The inner exception

ArgumentException

The parameter is incorrect. (Exception from HRESULT: 0x80070057 (E_INVALIDARG))

Things I tried so far:

• I've added all Extended WPF Toolkit libraries in the .vsixmanifest file as Assets, so now the libraries are added to the solution itself and they are marked with CopyAlways=True and BuildAction=Content, but the exception stays.
  
• I've cleared the bin/Debug and the C:\Users\userName\AppData\Local\Temp folders.
  
• I've reseted the VS Experimental Hive enviroment.
  

Nothing has worked out so far.

What do I have to do to get the Extended WPF Toolkit to work in a Visual Studio 2013 Package?

Thanks