Quickbooks backup to NAS drive problem

We have our Quickbooks file on the server.
Quickbooks is installed on all computers, and use the Quickbooks file on the server.
The backup location for the Quickbooks file, is on a NAS drive which is joined the domain.
The backup folder (on the NAS drive) is mapped to each computer, and Quickbooks points to the map.
When performing a backup when Quickbooks is open (File/Backup company/Create local backup/Next/Save it now), its perform a successful backup.
BUT when Quickbooks lunch a backup upon exiting the program it give an error.