Migration session hangs - DSA log reports "Common LDAP error 0x51. Server Down (empty)" (4265800)

Migration session hangs - DSA log reports Common LDAP error 0x51. Server Down (empty) with very little other information.
LDP test from DSA agent server to source & target DC/GC is successful.

Reviewing the Event log on the DSA server reveals the following:

Event Type: Error
Event Source: Configurator (HOSTNAME)
Event Category: (1)
Event ID: 4
Date: 25/11/2007
Time: 20:58:31
User: N/A
Computer: HOSTNAME
Description Server Down (Empty).

Restart the Quest Directory Synchronization Agent Configurator Service. If this doesnt help reboot server where DSA agent is installed.

Also make sure that in Agent Manager, DSA properties, preferred DC/GC names are resolvable from DSA machine.

Note:  It has also been found in some environments that specifying a preferred Domain Controller can alleviate the issue.  Especially if the error includes the domain of a child or parent not involved in the Domain Pair.  Setting the preferred Domain Controller bypasses the default behavior of utilizing a Domain Controller based on least cost.