.... and it fixed my problems !
/COPY

AT worked
/COPY

ATS didn't work
/COPY

ATSO didn't work
/COPY

ATSOU didn't work
With XP026 they all just work
.... when copying from one server to another in the same domain,
using a scheduled batch file.
I did give myself 'Log on as a service' privilege, but that doesn't seem to
be necessary.
I already had 'Manage Security & Auditing Logs' privilege, which someone
also suggested is necessary,
It's worth persevering to find Robocopy XP026. Maybe even worth buying the
Windows XP Resource Kit !