have you encountered this backup problem

OS: Windows 2003 SE
Backup media: Dell PowerVault 100T
Backup Application: Windows 2003 SE-NTbackup

If do a manual (non-scheduled) backup, the process works fine but when it is a scheduled normal backup, I receive this message.

“Cannot locate the specified media or backup device. This backup operation will terminate”

Microsoft site (technical support) has no preferred solution for this (I searched on their site) and this really makes me uncomfortable because of its ingenuinity.

Any solution would be a big help.