Help with "Go away, you don't exist" error message

I am receiving this error message from openssh when making an ssh  
connection.  Googling indicates that this message is usually received  
in response to a configuration problem, but I don't think that this  
is my situation.  Any other ideas?  Here's the deal.

I am running a program that calls for periodic connections, using  
openssh, between two Mac minis.  The client machine is running MacOSX  
10.5, and the server is running MacOSX 10.4.  Both are running
	OpenSSH_4.7p1, OpenSSL 0.9.7l 28 Sep 2006
as installed by Apple (not sure of the corresponding sshd version.)

A connection is attempted every five minutes.  Sometimes a connection  
is reliably established for several hours, E.g. twelve times an hour  
for six hours = 72 successful connections, say, and then the ssh  
connection will fail with this error message: Go away, you don't  
exist.  Once it fails in this way, all subsequent connection attempts  
also fail.

The server is not subject to heavy load, even sporadically.  The  
error message appears within seconds (according to the log)  
suggesting that it is not a timeout.

The server does not use PAM to authenticate connections (instead  
using a check against /etc/passwdck.)
The server appears to be correctly configured, as evidenced by the  
many successful attempts, and the fact that we've never seen an  
interactive connection attempt between these machines fail.   
Connections are made using server "authorized_keys" and "id_dsa" keys  
to avoid requiring interactive authentication.

Does anyone have suggestions as to how to track down this problem?   
Does it sound familiar to anyone?  I don't see anything in the docs,  
on the mailing list or by google that appears to match this issue,  
but perhaps I have missed something.  Pointers in the right direction  
much appreciated.

-- Dave

In case it matters, the periodic connections are initiated by cruise  
control (http://cruisecontrol.sourceforge.net/) and are to access a  
subversion (http://subversion.tigris.org) source code repository,  
using the svn protocol through an ssh tunnel (svn+ssh protocol.)  The  
reported error, however, comes from OpenSSH.