OK, I have this client that had failed backups and event viewer showed NTFS errors with event ID 55. The event log entry suggested that I "please run the chkdsk utility on the volume \Device\HarddiskVolumeShadowCopy1".
I performed the obligatory searches of Microsoft's KB database and asked Google, and the closest I got to an exact match was an unanswered call for help on Symantec's forums.
Here's the symptoms:
Backup failed and the indicated the following errors:
Could not access portions of directory System State\COM+ Class Registration Database.
Could not access portions of directory System State\Registry.
Event Viewer showed NTFS errors with event ID 55.
I could make a copy of unopened files.
"VSSADMIN LIST WRITERS" showed no errors.
CHKDSK /F showed no errors.
EventID.NET had a suggestion but it didn't seem to apply. Microsoft has a whole bunch of articles about VSS. The most drastic suggestion was 'do a backup, format and restore' -- except, of course, I couldn't do an online backup.
Anyway, in the end, here's what worked: I emptied the recycle bin, ran disk cleanup, and everything was fine with NTBackup again. Go figure.