ConfigMgr / SCCM 2007 Site Backup: ‘Error: VSS_E_WRITERERROR_RETRYABLE. Error Code = 0x800423f3’

I am trying to backup my Site Server using the built in maintenance task but when I check the “smsbkup.log” I see the following lines:

Info: Asynchronous BackupComplete finished.
 After BackupComplete SMS Writer status = WAITING_FOR_FREEZE.
 Error: VSS_E_WRITERERROR_RETRYABLE. Error Code = 0x800423f3
 04/16/13 00:24:30 - backup completed.
 Backup task completed successfully with zero errors but there could be some warnings, 
 AFTERBACKUP.BAT will be started if available in its predefined location

I have tried running “vssadmin list writers” from a Command Prompt to see if this clears the problems but although it lists the “SMS Writer” it shows the “Last error:” as “Retryable error”:

C:\>vssadmin list writers
 vssadmin 1.1 - Volume Shadow Copy Service administrative command-line tool
 (C) Copyright 2001 Microsoft Corp.
 Writer name: 'SMS Writer'
 Writer Id: {03ba67dd-dc6d-4729-a038-251f7018463b}
 Writer Instance Id: {74ed4f3d-268c-4a60-b0b3-efbb97c088fc}
 State: [2] Waiting for freeze
 Last error: Retryable error