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On our ISA system the following warning is logged:

Event Type: Warning
Event Source: Microsoft ISA Server Storage
Event ID: 21326
Description: The ISA Server VSS Writer encountered a failure while backing up 
the ISA Server configuration.

The system configuration is:

  • Windows 2003 Standard SP 2
  • Microsoft ISA 2006 SP 1

Backup agent: EMC NetWorker

Running vssadmin list writers returns

Writer name: "ISA Writer"
Writer Id: {25f33a79-3162-4496-8a7d-caf8e7328205}
Writer Instance Id {f958b2f0-7ad1-4326-9ddc-51aee0ef24c4}
State: [8] Failed
Last error: Retryable error

I'm looking for any hints on what's going wrong here and how I can fix this.

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up vote 0 down vote accepted

The cause for this issue is that the function CryptSignHash passes AT_SIGNATURE as a parameter. This will fail if the "signature" key pair / certificate is missing. See

The only supported solution is to set up the machine and ISA Server from scratch.

An unsupported solution ist to create a new key container for ISA.

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