Cannot add new files to iTunes

Since a week or so, I've been unable to copy music files from a folder (i.e. Downloads) to my iTunes music folder via iTunes with drag and drop. Every time I get an window saying that iTunes failed to copy the files to my disk.

The window suggest that I verify the Permissions of my music folder. I made sure that it's Read and Write for me and for the other accounts on my computers.

Any help would be greatly appreciated.

P.S. For additional information sake, my music folder is located in my public folder.