VSS Unexpected error querying for the IVssWriterCallback—Access Denied

I'm having trouble with VSS based backups of my Exchange server. I'm getting the following error on the start of the backup:

Volume Shadow Copy Service error: Unexpected error querying for the IVssWriterCallback interface.  hr = 0x80070005, Access is denied.
. This is often caused by incorrect security settings in either the writer or requestor process.


Operation:
   Gathering Writer Data


Context:
   Writer Class Id: {e8132975-6f93-4464-a53e-1050253ae220}
   Writer Name: System Writer
   Writer Instance ID: {6524807c-855e-4b4f-9295-dab2ac1e1948}

The service performing the backup is running as Local System, so I don't understand why it wouldn't have access. I've checked group policy and DCOM; there's no antivirus on the system and the firewall is disabled.

Edit: The backup software is CloudBerry Enterprise Backup. I believe it uses the built-in Exchange Information Store writer, as I didn't see anything that looked like a third party writer when I ran vssadmin list writers. It's quite possible that I could have missed it, however.