I manage 2 SBS 2008 servers that are having the same issue. I will post error below. Shadow copies are not running at the time the backup kicks off. The backup runs every 4 hours. What's strange is that the backup appears to run successfully. There are 75+ occurences in the event log every day. I have not had much like finding a solution online.
1. Please find attached document and reregister all mentioned DLL then restart Server after restart execute "vssadmin list writers" command to check the state of VSS writers.
Note: It is recommended to reboot the server after every patch installation.
It solved it right away. Thank you!
I myself have reregistered DLL's for VSS on a number of SBS 2008 machines without incident.