Hi Severin,
I don't know if your problem is well-known or not (i.e., can Google shed any light or not?). But I think the last time you had this problem, I suggested that you make the files available to one or some of us so that we can investigate/ Unfortunately, on that occasion, you had already deleted the files. All I can imagine is that your files might have names that Windows can't handle (e.g., if you had a file named "Image 16/02/2006.jpeg" -- Windows probably can't handle the presence of slashes in the filename).
