NFS server fails after upgrade to lenny

[hendrik@topoi.pooq.com]

I upgraded the server on my LAN to lenny a few months ago (after 
making a backup of the etch system (everything except user file 
systems)) in another partition.

Now when I run lenny on the server, an NFS mount from a client machine
fails with the message (hand-transcribed from screen; there may be 
typos)

lovesong:~# mount /farhome
mount.nfs: mount to NFS server '172.25.1.11' failed: RPC Error: Program
not registered
lovesong:~#


But when I reboot the server to etch using the etch I saved in the 
backup partitions:

lovesong:~# #mount /ferhome
[514.141911] RPC: Registered udp transport module. [514.143371] RPC:
Registered tcp transport module.
lovesong:~#

So the upgrade seems to have damaged something.  Where should I look?

And, by the way, I have installed nfs-common.  That one seems to be 
there in the etch system, too.

I'm currently running the server using my backup copy of the old etch
system.  I'd like to be able to move to lenny (and be ready for the 
now-looming transition of stable to squeeze), but right 
now it's infeasible.

My users are all out-of-town for a few days, so it's now easy to boot 
lenny to diagnose the problem.

-- hendrik