I'm setting up an Ubuntu NFS server with a Solaris 10 client. The basic configuration looks okay to me, and it was also working for some time. I'm getting an "RPC: Authentication error" message on the client.

server

/etc/exports:

/export/opencsw-future 192.168.3.0/24(rw,nohide,insecure,no_subtree_check,async)
/export/opencsw-current 192.168.3.0/24(rw,nohide,insecure,no_subtree_check,async)

$ ls -ld /export/opencsw-current
drwxr-xr-x 7 maciej maciej 4096 2012-02-05 14:55 /export/opencsw-current

client

$ grep opencsw /etc/vfstab
foosrv:/opencsw-current - /export/opencsw-current nfs - yes -
$ sudo mount /export/opencsw-current
NFS compound failed for server foosrv: error 7 (RPC: Authentication error)
(...repeated...)
nfs mount: mount: /export/opencsw-current: Permission denied

My server host name resolves to both IPv4 and IPv6 addresses.