Restart to solve NFS Unknown error 521

I just had Unknown error 521 while mounting NFS version 3 on a Debian server after a reboot. Solving it took me some time but the solution was very easy.

My setup is NFS server on storage server running Debian and a NFS client on another Debian machine where connecting to the NFS server. After a long period of time i made a reboot on my client machine and got mount.nfs: Unknown error 521

My /etc/fstab is as follow:

1.2.3.4:/srv/nfs/web1/mail /var/mail    nfs      nfsvers=3,rw,async,hard,fsc,intr  0     0

I could mount it running version 4 but in verion 3 i got the error.

When i tried with -v option i got the following output:

mount.nfs -o vers=3 1.2.3.4:/srv/nfs/web1/mail /var/mail -v
mount.nfs: timeout set for Mon Sep 22 17:25:16 2014
mount.nfs: trying text-based options 'vers=3,addr=1.2.3.4'
mount.nfs: prog 100003, trying vers=3, prot=6
mount.nfs: trying 1.2.3.4 prog 100003 vers 3 prot TCP port 38467
mount.nfs: prog 100005, trying vers=3, prot=17
mount.nfs: trying 1.2.3.4 prog 100005 vers 3 prot UDP port 47766
mount.nfs: mount(2): Unknown error 521
mount.nfs: Unknown error 521

The simple solution was to restart the NFS server service.

On my NFS Server i did:

/etc/init.d/nfs-kernel-server restart

After that i got my NFS client to connect with version 3 on the NFS server without any errors.

mount.nfs -o vers=3 1.2.3.4:/srv/nfs/web1/mail /var/mail -v
mount.nfs: timeout set for Mon Sep 22 17:28:15 2014
mount.nfs: trying text-based options 'vers=3,addr=1.2.3.4'
mount.nfs: prog 100003, trying vers=3, prot=6
mount.nfs: trying 1.2.3.4 prog 100003 vers 3 prot TCP port 2049
mount.nfs: prog 100005, trying vers=3, prot=17
mount.nfs: trying 1.2.3.4 prog 100005 vers 3 prot UDP port 54305

I hope this will help someone solve this issue faster then me.