The error VSS_E_SNAPSHOT_SET_IN_PROGRESS may be seen in a Disk Image or File and Folder backup log.
Additionally in the backup and VSS log you may see the following additional information:
VSS_E_SNAPSHOT_SET_IN_PROGRESS indicates that another application has an open volume snapshot on the volume being imaged. It is only permitted to have one open volume snapshot on the same volume at any one time. This could be caused by another backup product or by Windows updates. If backup runs at the same time as Windows updates then this could be the cause of the problem.
If you are running another backup program that also uses Microsoft Volume Shadow copy Service (VSS) then ensure that any scheduled backups to not conflict. An example of of another VSS backup program is Carbonite.
If your backups run at the same time as your Windows update schedule then either schedule your Macrium backups for a different time or change your Windows update schedule.