The following errors appear on the NFS server, could you please advise how I can fix this?

Details:

System: CentOS release 6.4, NFS: nfs-utils-1.2.3-36

# cat /etc/idmapd.conf

[General]
Domain = domain.com

[Mapping]
Nobody-User = nobody
Nobody-Group = nobody

[Translation]
Method = nsswitch

Sep  3 08:25:28 snode1 rpc.idmapd[1382]: nss_getpwnam: name '0' does not map into domain 'domain.com'
Sep  3 08:25:29 snode1 rpc.idmapd[1382]: nss_getpwnam: name '500' does not map into domain 'domain.com'

EDIT: 03 Sep 2013 10:41

Please note that I'm using NFSv4 and these errors appear on NFS server only (not NFS clients).

Server:

# cat /etc/sysconfig/nfs

MOUNTD_NFS_V2="no"
MOUNTD_NFS_V3="no"
...
RPCNFSDARGS="-N 2 -N 3"

Clients:

# cat /etc/fstab

server:/     /data  nfs4    defaults,hard,intr,timeo=15,_netdev,noatime,nodiratime,nosuid    0 0