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).