I am using the default config while adding the specific directory with nginx installed on my ubuntu 12.04 machine.
server {
#listen 80; ## listen
In my case, the folder which served the files was a symbolic link to another folder, made with
ln -sf /origin /var/www/destination
Even though the permissions (user and group) where correct on the destination folder (the symbolic link), I still had the error because Nginx needed to have permissions to the origin folder whole's hierarchy as well.