Make sure the DomainBVeritas account has change permissions on the NAS CIFS share.
Try logging on to a DomainB PC with the Veritas account and attach to the NAS. Attempt to copy a file to the share using a CMD.EXE prompt. This may give you a better clue of the issus.
If that is all set up properly, its definitly a trust issue. Maybe try re-establishing the trust.