This error message is returned by Microsoft VSS when two or more processes create a "collision" in requests for VSS snapshots.
Microsoft limits the number of processes requesting a VSS snapshot to a single process at a time. SureSync AOFS (Advanced Open File Support) uses VSS to copy locked, open files. If another process happens to also be using VSS at the same moment in time, this error may be returned to the process which failed to obtain the requested snapshot.
This often occurs in SureSync when you have two or more Schedules or Real Time Monitors running which use AOFS and have requested a snapshot of a locked open file at the same moment. One Job will be allowed to copy the locked open file and the other will receive this or a related error message - VSS_E_SNAPSHOT_SET_IN_PROGRESS - and the request will fail.
To resolve this problem, try to avoid running two or more SureSync Jobs using Advanced Open File Support on the same volume at the same time.