Bug Snapshot failure leaves VSS snapshot set in progress, causing subsequent snapshot attempts to fail.

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Environment info

• Duplicati version: 2.3.0.4 - 2.3.0.4_stable_2026-07-09
• Operating system: Windows 11
• Backend: WebDAV to Koofr.com
• Snapshot option enabled: Auto
• Other affected backup software: Veeam Agent for Microsoft Windows Free edition.

Description

During testing, the same backup job works correctly when snapshot-policy is set to "required". When the policy is set to auto, snapshot creation fails much more frequently. I have not identified what causes the "auto" to fail more often.

When snapshot-policy is set to auto, Duplicati attempts to create a Windows Volume Shadow Copy Service (VSS) snapshot. If that snapshot attempt fails, Duplicati continues the backup without using a snapshot, as expected for the auto policy. However, the failed snapshot attempt appears to leave the VSS snapshot set in an incomplete or in-progress state.

After this occurs, no other application can create a VSS snapshot. This affects both Duplicati and other VSS-based backup applications, including Veeam Agent for Microsoft Windows.

Subsequent snapshot attempts fail with:

VSS_E_SNAPSHOT_SET_IN_PROGRESS Code: 0x80042316

The issue is cleared by rebooting Windows.

Further testing indicates that the actual trigger is a failed snapshot attempt while snapshot-policy=auto is enabled.

Steps to reproduce Configure a Duplicati backup job with snapshot-policy=auto. Run the backup under conditions that cause the VSS snapshot attempt to fail. Allow Duplicati to continue and complete the backup without a snapshot. Run another Duplicati backup that attempts to use snapshots, or run another VSS-based backup application such as Veeam Agent for Microsoft Windows. Observe that the new snapshot attempt fails with VSS_E_SNAPSHOT_SET_IN_PROGRESS. Reboot Windows. Run the snapshot-based backup again. Observe that VSS snapshot creation works again.

The problem reported here is that Duplicati does not appear to clean up the VSS operation after that failure.

Actual result

When snapshot creation fails under snapshot-policy=auto, Duplicati silently continues the backup without a snapshot.

However, the failed VSS operation appears to remain open or incomplete. VSS continues to report that a snapshot set is already in progress.

As a result, subsequent snapshot attempts by Duplicati or other VSS-based applications fail with:

VSS_E_SNAPSHOT_SET_IN_PROGRESS Code: 0x80042316

Duplicati does not appear to log the original snapshot failure when the policy is set to auto, making the cause difficult to identify.

Expected result

When snapshot creation fails under snapshot-policy=auto, Duplicati should:

Abort and clean up the failed VSS snapshot operation completely. Continue the backup without a snapshot. Leave VSS available for subsequent snapshot operations. Record an informational or debug-level log entry indicating that snapshot creation failed and the backup continued without a snapshot.

Impact

After the failed Duplicati snapshot attempt: Duplicati can no longer create another VSS snapshot. Other VSS-based backup applications cannot create snapshots. Backup jobs that require VSS fail. The user may not know that the original Duplicati snapshot attempt failed because auto suppresses the failure message. Workaround

The VSS state can be cleared by Reboot Windows.

After the stale VSS state is cleared, snapshot-based backup applications work again. Veeam error

The following error was recorded by Veeam Agent while the failed Duplicati VSS operation was still in progress:

7/10/2026 3:32:21 PM :: Creating VSS snapshot Error: Failed to create snapshot: Backup job failed. Discovery phase failed. Cannot start empty snapshot set. VSS error: VSS_E_SNAPSHOT_SET_IN_PROGRESS. Code: 0x80042316