This article addresses the Windows backup error: VSS Export error occurred mid-transfer - falling back to STC.
The ShadowSnap agent attempted to take a VSS backup but was unable to do so, due to slow or busy VSS writers on the protected system.
By default, the ShadowSnap agent attempts to perform a crash-consistent STC backup upon a failed VSS attempt.
This error is not necessarily indicative of a failed backup, just that the backup could not be taken via VSS. The backup agent will retry up to 3 times in a given backup request to take a backup. The engine will typically switch over to crash-consistent backups on the first failed VSS backup attempt and will attempt another VSS backup upon the next scheduled backup.
When troubleshooting, please see the following article that provides further information about the VSS writers and what to do when they aren't working properly.