We're having the exact same problem with Server 2008 Standard 64-bit. I cannot successfully complete any backups with Windows Server Backup in its default configuration.
Event Viewer also displays error code '2155348129' but our System parition is correctly marked as Active already.
However, If I disable SQL Server VSS Writer in services.msc, the Backup successfully completes. I decided to stop this service when I noticed that its entry in 'vssadmin list writers' was marked as 'non-retryable error'