Cannot access User information using Address Book via VPN or through named LDAP directory

I have an Open Directory Server set up at LDAPServer.company.private.   on a Mac OS X 10.5.3 Server.   I am using simple authentication because this directory is only accessbile locally (and to reduce the number of variables in the problem).

I can access User data via Address Book locally, on the server, via Group- Directories, Directories - Directory Services.

I cannot access User data via Address Book locally, on the server, via a named directory which I set up through Address Book - Preferences - LDAP to point to LDAPServer.company.private with seach base:  cn=users,dc=ldapserver,dc=company,dc=private

I can access User data via Address Book on the local subnet on my LAN from a Mac OS X 10.4 client workstation via Group-Directories Directories- Directory.

I cannot access User data via Address Book on the local subnet on my LAN from a Mac OS X 10.4 client workstation via a named directory which I set up through Address Book - Preferences - LDAP to point to LDAPServer.company.private with seach base:  cn=users,dc=ldapserver,dc=company,dc=private

I cannot access User data via Address Book over a PPTP VPN connection (the LDAP Server is also DNS and VPN server neither through Group-Directories Directories-Directory nor through a named directory which I set up through Address Book - Preferences - LDAP to point to LDAPServer.company.private with seach base:  cn=users,dc=ldapserver,dc=company,dc=private.

Over the VPN I can ping the LDAPServer.company.private server by name.  I do have GRE protocol, PPTP port 1723, LDAP port 389 and ALLOW rule 65534 on the firewall of the server, which is directly connected to the Internet.

I would appreciate guidance on why I cannot access User data via Address Book using a specified LDAP directory, even on the local server itself, even though the data resides in something called "Directory" whose configuration parameters are completely unknown.

It is maddening to attempt to search words like "Address Book", "LDAP",  and "Directory" as these are practically stopwords.

It seems like the search base must be the problem preventing local subnet access using a specified LDAP directory.  If I can get that working, I have have some hope of getting the VPN working.