I've just tried it again and it now works.

We had trouble installing a mailbox antivirus product a few weeks ago and
this highlighted an issue with our MDAC installation. MDAC was updated and
repaired, going from version 2.6 to 2.8, and we think this is what resolved
the issue. MDAC, it appears, had always been improperly registered and
installed, ever since we upgraded the server o/s from 2000 to 2003 and
performed a clean install of Exchange.

My colleague, who resolved the issue with MDAC, used the following article
and the software it refers to:

http://www.macropool.com/en/download/repair_mdac.html

I hope this is of use.


-----original post:

From: Paul Anderson (paul.anderson@eaifhe.REMOVE_NOSPAM.ac.uk)
Subject: Can't Create Appointments in OWA 2003



View this article only
Newsgroups: microsoft.public.exchange.admin,
microsoft.public.exchange.clients, microsoft.public.exchange.misc,
microsoft.public.exchange.setup
Date: 2004-01-15 06:25:08 PST

We have Exchange Server 2003 and Outlook 2002. Our users can create
calendar entries using Outlook, but not using Outlook Web Access. When they
click 'Save and Close' in the New Appointment window, they get the message
'The appointment can't be opened'. They can access all other Exchange
mailbox content normally using OWA. They can even open existing calender
items created using Outlook, but they can't amend them. They can copy an
e-mail into the Calendar from Inbox.

Again, everything works normally in Outlook, but not OWA.

--
Paul Anderson
IT Systems Support
East Antrim Institute of Further & Higher Education