"Can't open the item" error after Find in Outlook 2003

I have several different Exchange mailboxes mapped to my Outlook. When I run
a standard search on one inbox, the results come back fine (in the main view)
with a preview in the Preview Pane, as expected.

However, running a standard search on an inbox in a different mailbox yields
a list of results but they cannot be viewed in the Preview Pane (there's a
message along the lines of "Cannot view in Preview Pane, please open the item
to view its contents"). However when I double-click on the e-mail, a prompt
comes up saying "Can't open the item".

Does anyone have a firm resolve for this? From my searching of Forums, it
seems like people have reported the issue previously. FYI... I an running
Outlook 2003 with SP2 and no Cached Exchange Mode. I read somewhere this may
be to do with Custom Form settings?

Any help much appreciated. Thanks.