...with the source VSS and the event ID 8194. This was causing my backups from a client computer to WHS to fail consistently. This is a security/permissions issue where the Network Service does not have permission to activate a COM service. I spent about two hours searching for the answer until I found this from http://help.remote-backup.com
The second (preferred) way to work around the issue is to make an adjustment to the default COM service activation permissions - allowing Network Service (and possibly Local Service) user account(s) to activate the IVssWriter callback interface. This method has the benefit of permanently fixing the issue in one place and allowing the VSS Writer service(s) to run at the privilege level that Microsoft intended. Any COM object accessed (by a process running as Network Service) still has the ability to enforce security restrictions it so chooses.
Make this fix by doing the follow: Start | Run | dcomcnfg. This brings up the Component Services application.
On the left pane navigate to Component Services | Computer | MyComputer.
Right click on MyComputer and select properties.
Select the COM Security tab and select the Edit Default button under Access Permissions.
Use the Add... button to add the "Network Service" account to the permission list.
Verify that ONLY the Local Access box is checked and click OK.
Close Component Services.
A reboot is then required to make the requested changes to COM Security.