That can't be true how much time I wasted on this problem.
This could be helpful for everyone: By default IIS Express is launched in 32-bit mode in Visual Studio 2013 Express Web. You can verify this behaviour in the task-manager. There is there "*32" annotation for the iisexpress.exe process.
It should be clear that iisexpress.exe can't load a 64-bit dll in 32-bit mode. So you first have to switch the iisexpress.exe to 64-bit mode.
You can do that under the menubar: Tools / Options / Project and Solutions / Web Projects / Use the 64 bit version of IIS Express for web sites and projects.
This could be helpful for everyone: By default IIS Express is launched in 32-bit mode in Visual Studio 2013 Express Web. You can verify this behaviour in the task-manager. There is there "*32" annotation for the iisexpress.exe process.
It should be clear that iisexpress.exe can't load a 64-bit dll in 32-bit mode. So you first have to switch the iisexpress.exe to 64-bit mode.
You can do that under the menubar: Tools / Options / Project and Solutions / Web Projects / Use the 64 bit version of IIS Express for web sites and projects.