I tried that, too. I get this when I try it on a share:

H:\working_files\win32>xcacls G:\IG-S
G:\IG-S <Account Domain not found>(OI)(CI)C
        BUILTIN\Administrators:(OI)(CI)F
        DOMAIN1\Domain Admins:(OI)(CI)F
        AD-DOMAIN\LabOps Admins:(OI)(CI)F
        NT AUTHORITY\SYSTEM:(OI)(CI)F
        BUILTIN\Users:(CI)R
[download]

So I did a google search, and found this link and this link, in addition to this link at the Microsoft knowledgebase, which seem to point to the unreliability of xcalcs. Although the knowledgebase article says it's only for NT 4, we're running Win2k, with all the needed patches.

So maybe it's something wrong with the OS itself ...