VSS event log error every three hours (System.Writer Access denied)

In my event log (Windows Server 2012) I see this error every three hours, going back a very long time:

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: {7d2e834e-3414-492a-9009-514581a351ee}

A search showed other people having trouble with their third-party backup software that uses VSS, but the suggestions haven't worked for me.

Here is what I've tried.

1. Used vssadmin list writers in a DOS window to show the VSS service statuses. This showed three services awaiting completion. I then re-started them manually and ran the third-party backup software manually. The services showed "stable - no error" on completion, but on the three hour mark the errors returned and the status switched back to "awaiting completion". This makes me think the problem lies outside the backup software.

2. Re-registered SqlVDI as per this answer, but no luck.

Can anyone cast any light on what might cause this, and how to prevent it? Perhaps there's a way to view what is triggering a scheduled event every three hours?