back-ldap and saslauthd err=4

[Andrew Reilly <andrew.reilly@gmail.com>]

My organization has distributed our DIT across several openldap master
servers.  To provide a view across the whole tree we have several
"central slaves" where all the masters are replicated to, and these
are fronted by a proxy cache using back-ldap for each central slave. 
This has been working quite well.

Recently I have been integrating an application that only supports
LDAP authentication via saslauthd.  Everything was humming along until
I have encountered an interesting error.  When I point saslauthd
directly at an ldap directory whether it is a master or a slave it
works, but if I point it at a back-ldap instance the result is an
err=4.  Now, from my reading err=4 occurs when a search exceeds the
configured number of returns but the search being performed by
saslauthd only returns one entry.  If I preform the exact same search
via ldapsearch against the ldap-back instance it works.

Any idea on what might be causing it, or how I might gather any useful
information on the cause?

TIA,
andrew