NFS V4 mount fails - Server side says idmapping failing
 

I have two machines on the LAN running NFS Version 4. When the first is booted 'rpc.idmapd' is started as part of the init process (/etc/init.d/nfs-common). Also, 'nfs-kernel-server' starts the server and exports the filesystem(s), with the root exported w/fsid=0.

When the second machine is booted it tries to mount the exported directories and I get a message on the server:

kernel: nfsd: nfsv4 idmapping failing: has idmapd not been started?

Well, yes it has been started. The mounts appear in the client (second) machine mount table, but I can't write to the remote directory
(it's a backup partition on an external drive connected to the server).

When I try to start a backup job using pdumpfs, the server puts out the following message:

rpc.idmapd[pid]: nss_getpwnam: name '0' does not map into domain localdomain

This had been working for about a year and a half, and it took me some
work to get NFSV4 working, so I'm perplexed about the current problem.

Any ideas about how to go about debugging and/or fixing this? It's driving me crazy.