try this:
open Terminal (in /Applications/Utilities) and enter the following commands :
rm -rf "~/Public/Drop Box"
mkdir "~/Public/Drop Box"
chmod 733 "~/Public/Drop Box"
touch "~/Public/Drop Box/.localized"
rm -f /Library/Caches/com.apple.LaunchServices.*
(this will delete and recreate your drop box fodler with the right permissions, and delete the Finder cache).
Log out and login back before trying to copy the file in the dropbox again.