Thank you for your quick answer.
I understand Windows Phone 8.1 is not officially supported, so thank you very much for giving me support about it.
After reading your answer, I tried to create a solution for Windows Phone only (.NET Framework 4.5.3) and it works. So the beta release is OK for Windows Phone project alone in solution.
Knowing that, I created a new Shared Assed Project (SAP) Xamarin Solution. As there is no PCL, only code is shared between platforms projects, this should be the same as a Windows Phone alone solution.
References added to my Windows Phone project are "Portable Class Library for Universal Apps".
I can compile and run on Android, but when I try to compile the Windows Phone project of the solution, here is the error on build process:
App.xaml(1,1,1,1): error : Cannot resolve dependency to assembly 'System.Runtime.InteropServices, Version=18.104.22.168, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' because it has not been preloaded. When using the ReflectionOnly APIs, dependent assemblies must be pre-loaded or loaded on demand through the ReflectionOnlyAssemblyResolve event.
Do you think there is a solution?
Thank you in advance for your support.