Windows Backups Failing with Associated VSS 8193 Error. In the Application Event Log was a prominent VSS error saying something about a ConvertStringSidToSid function which failed.

Here’s the original text for better searchability:

This was followed by a 521 Event from Windows Backup telling you that the Backup operation failed.

The hint was already in the event details from the VSS error:

ConvertStringSidToSid(S-1-5-21-2828234547-00000000000-1509355905-1125.bak)

Can you spot the SID with the BAK extension at the end?

Looked at the ProfileList regkey on that machine.

And here it is:

Followed KB947215 and ultimately removed the Profile with the BAK extension and started another Windows SystemState Backup, which succeeded without any errors.

Here’s an automated way to clean up the old profiles from that list.

Script to fix issues described in KB947215 (PowerShell)