Mixed mode assembly is built against version 'v2.0.50727' ... error on native C++ / Visual C++ / C# projects

I am trying to instantiate a C# class in a native C++ .exe project using a Visual C++ wrapper.

The C# project targets .Net 3.5 framework, and this cannot be changed. Both C++ and Visual C++ projects are build using the v100 compiler.

But I still get the Mixed mode assembly is built against version 'v2.0.50727' of the runtime and cannot be loaded in the 4.0 runtime without additional configuration information.

I have also tried to add

<TargetFrameworkVersion>v3.5</TargetFrameworkVersion>

to the <PropertyGroup Label="Globals"> elements of the .vcxproj files of the C++ projects, but still the error.

If somebody has a an idea, he/she will be welcomed...