fence_apc fails ("Failed: Unable to obtain correct plug status or plug is not available")

Issue

• 8 node Red Hat Enterprise Linux 5.5 cluster, using gfs(1)
• 2 apc fencing devices used, one for half the nodes (1-4) and one for the other half (5-8)
• attempting to fence nodes on the second device (nodes 5-8) fails - resulting in gfs/lvm lockup
• attempting to fence using fence_apc shows the following error:

    American Power Conversion               Network Management Card AOS      v1.1.6
    (c) Copyright 2002 All Rights Reserved  Rack PDU APP                     v1.0.1
    -------------------------------------------------------------------------------
    Name      : Unknown                                   Date : 09/18/2006
    Contact   : Unknown                                   Time : 18:54:34
    Location  : Unknown                                   User : Administrator
    Up Time   : 75 Days 8 Hours 49 Minutes                Stat : P+ N+ A+

    Switched Rack PDU: Communication Established

    ------- Control Console -------------------------------------------------------

         1- Device Manager
         2- Network
         3- System
         4- Logout

         <ESC>- Main Menu, <ENTER>- Refresh, <CTRL-L>- Event Log

        Failed: Unable to obtain correct plug status or plug is not available