Found this from Novell, it mentions Backup Exec but may be applicable here also. At least worth checking to see if they are compressed files on an NSS volume:
TID10071768
Error: fffdffcd when attempting to backup compressed files on a NetWare 6 NSS volume.
SMS Error: fffdffcd (file: mopen.c, line 349) Error Text: "(TSA600.NLM 6.0 306) A data stream cannot be opened
Insufficient privilege errors being reported from Backup Exec
change
Attempting to perform a backup of NSS version 3.01 compressed files is failing.
fix
Apply the following updated files:
1) COMN.NSS Version 3.01 Friday, May 03, 2002 or newer.
This updated file has been included in Support Pack 2 for NetWare 6.
To download the Beta Consolidated Support Pack 8 please go to the beta download side:
http://beta.novell.com/public.jsp
2) NW6NSS1B.EXE or newer