The file is corrupted (0x4000D)| Tag= 0xC847B314771C5EB3

I am using Acronis True Image 9.1 under XP pro (fully updated)
I make regular backup of 2 partitions sequentially on time of day to a NAS.
The first runs to completion without error.
In the last week or so the second longer one (from a different partition) with about 40G of 50G used goes through the backup process with max compression, then starts the verification, and emits the very helpful error message:

The file is corrupted. (0x4000D) Tag = 0xC847B314771C5EB3
and displays
Operation has completed with errors.

I can make selected recovery from the archive.

I have run disc check on the volume and it claims no errors.

I have emptied the recycle bin.

Please can anybody suggest the reason for this as I am nervous if there is (as appears to be the case) some hidden error .

David Osborne