It looks like App42 SDK for WinRT v 2.1 (appears to be the latest one) is not signed with a strong key.
Whether we like it or not (I actually don't like it, but have to live with it :) ), Microsoft requires that, if the app is signed, than ALL its components must be signed, otherwise the application crashes as soon as it tries to load an unsigned DLL . For that reason I cannot sign my app.
The fix in Visual Studio is pretty straighforward: you just create a strong name key file in Signing section of project properties and tick "Sign" box. As far as I understand it, this will allow the App42 SDK library to run with both signed and unsigned apps.