if you get this error it might be because the permissions for the folder on the server need to be set to 755

like this:

# on the server
chmod 755 /path/to/shared/folder

full story:

# on the server
mkdir -p /path/to/shared/folder
chmod 755 /path/to/shared/folder

vim /etc/exports
#                        ip of client allowed to access it
exportfs -a;

# client
mkdir -p /mnt/nfs/folder

# try to
#         ip of the server
mount -vv /mnt/nfs/folder