The error message indicates that the size of the datad.nv file does not agree with the internal file size marker.
The datad.nv file was not open when backed up. If the books where open when you had tried to back them up you would have received an error message along the lines “DATAD.NV access denied” or in use by another process.
Run check books on the books (from the NewViews shell) and if result is reorganization is necessary then Reorganze the books. If the result is Heap Overflow or “Ram Buffer locked while Freeing”, then the books must come in for data recovery.
Note: When you run check books, pay attention to the number of accounts. If the number of accounts encountered does not match the number of accounts on file then reorganization is possible but the end result may be messy.