Normally I can add classes to my ASP.NET project, but it is very weird and now I am getting this error when adding a class:

Also my project doesn't use any namespaces anymore. I created another clean ASP.NET project, but the same issue appears there too...
When I click "Yes" (Ja) then the class will be added to the project in a folder names App_Code, without namespaces, and when I click "No" (Nee) then the class will be added to the normal project folder, also without namespaces.
Anyone who can help me with this problem? I can't create new instances of my classes.