The answer to this was just given by Brian Candler in another thread,
but I'm putting it here, too, for Google's sake, for future people
having this problem:


=======  THE SOLUTION :  (WORKS!)  ==========
The client side machine will read its own system
hostname (as shown by 'hostname' on many Unix systems), and pass that across
to the server. At the time it wishes to connect back, the server will then
try to resolve that name back to an IP address.

So it's important that 'hostname' contains a real hostname and your DNS is
working, or at least at the server side you have an entry in /etc/hosts for
the client host.
=========  (THANKS BRIAN!) ===============