This error occurs because a backup or restore session state in the VSS writer is not cleaned up correctly. The VSS writer infrastructure creates session-state objects for each backup and restore session in which a VSS writer participates. Under typical circumstances, the writer session-state objects are cleaned up when they are not being used. Normal session state cleanup occurs under the following circumstances:
- The backup application sends a BackupComplete response and then checks the writer status by sending a GatherWriterStatus.
- The backup application sends a PostRestore response and then checks the writer status by sending a GatherWriterStatus query.
- The writers receive the OnAbort event callback for the session. The OnAbort event callback is invoked when the backup session is explicitly failed by the backup application, by the writer or, by the VSS infrastructure.
The VSS writer infrastructure performs periodic garbage collection of the remaining session states. The infrustructure then logs the previous event log for each session-state object that is older than two days. The event log is intended to help identify abandoned sessions in the writer that may indicate a misbehaving backup application. You can see several similar events in rapid succession from multiple writers that indicate that there was a series of incomplete backup or restore sessions. This behavior is most frequently seen in test environments.