'Backup' function does not work

[ss]

My ''Backup' (ntbackup.exe) function does not work anymore.
I receive the following error message:
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Backup Status
Operation: Backup
Active backup destination: File
Media name: "20060120.bkf created 1/20/2006 at 5:05 PM"
Error returned while creating the volume shadow copy:80042301
Aborting Backup.
The operation did not successfully complete.
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'Backup' previously worked perfectly Virus,malware scans are OK.
My system works perfectly except for Backup.

Any advice appreciated