Hi,
This was another one of those kickself problems. I had my eureka moment this morning soon after I posted the question. Although I had copied the user id in Active Directory and so all of the settings should have been the same as the userid I had copied from, the new userid had no mailbox permission for itself. So, the new userid was not permitted to pick up the new mailbox.
I corrected this oversight and the thing now works... Hooray!
As to why the same version of Outlook 2000 got to a different error message when run under different operating systems, I haven't a clue. But it works now on both platforms. So that's OK by me!
Thanks. I hope that this explanation may help others trapped in the same predicament.