C# PCL+UWP - Failed to load "System" assembly

I've got a project which has a Portable Class Library as "back-end" and a Universal Windows app for "front-end". Here is my repo at GitHub: huming2207/Rmiter

My target settings for the PCL library is "portable46-net451+win81", also the screenshot shows below:

When running the app it stuck at the beginning, showing a System.IO.FileLoadException from the backend PCL library, as below:

System.IO.FileLoadException: 'Could not load file or assembly 'System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)'

...and here is the screenshot (in case if someone misunderstand my words):

But meanwhile I can see there was a "System.Runtime.dll" loaded when the app starts. But why it still throw exceptions later on?

I've searched and checked for quite a lot of similar issues and workarounds. But it seems that neither of them help at all.