Finally, after cleaning my project, I can compile again from uproject, but the main question of this thread is still happening.
And yes, is the same with new projects, this is what I do exactly:
- Open the .sln of the project
- right click on the solution → Debug → Start new instance (the configuration of the solution is Development Editor and Win64)
- when the editor is open, go to File->Package project->Windows-> 64bits
Anyway, I think this is caused by something with the update of Visual Studio 2015 (got VS 2015 community with ReSharper).
Thx for all your help 