NFS access failed for server xxx: error 7 (RPC : Authentication error

["GrishNa Chong" <grish () dusit ac th>]

I have server that share  /var/mail
client1 can be mount /var/mail perfectly
client 2 can be mount too but can not access to /var/mail

cd  /var/mail
NFS access failed for server xxx: error 7 (RPC : Authentication error)
cd :  permission denied

what happen ?

client 1 is "nis+ client"
client 2 used to be "nis+ master server"
but now it has change to be "nis+ client"