As I've already said: It can only be the server(-side) caching. Since revoming of the browser cache and Ctrl+F5 don't help.

But now I think, it might be an issue of the virtual machine. Because when I edit the files directly from the VM* and not in my hosst system, sometimes I can see the changes in the Browser.

* The files are saved in a shared folder, so I can access to them both -- from the host and also from the guest system.