Cannot backup 'System State' - Could not access portions of directory System State

Hello newsgroup,

I just tried to backup the System State of our DC with NTBackup and got the
following error(s):

--- < Backup Log ---
Error: Could not access portions of directory System State\COM+ Class
Registration Database.
You may not have permission to open the file, or the directory may be
missing or damaged.
Please contact the owner or administrator.

Error: Could not access portions of directory System State\Registry.
You may not have permission to open the file, or the directory may be
missing or damaged.
Please contact the owner or administrator.
--- Backup Log > ---

While NTBackup is running, event log will log the following error:

Source: NTBackup
Event ID: 8001
Type: Error
Description:
End Backup of 'System State' 'Warnings or errors were encountered.'

Verify: Off
Mode: Replace
Type: Copy


vssadmin lists all writers without any errors.

VSS startup type is set to "Manual" (default). While NTBackup is running,
VSS will be started.

Some details about the system:
It is a Windows 2003 EE SP2 machine with R2 addon, configured as DC.


Regards,
Thomas