same problem here. ZCO Connector 5.0.18 works perfectly well, all later versions (5.0.19, 5.0.20, 6.0.2) are not working. they all raise the error message:
Unable to open your default e-mail folders.
This only happens on Outlook 2003 on XP (SP2). Outlook 2007 is fine.
I am using a "fresh" Zimbra account. There is only one mail in the Inbox an one mail in the Sent folder. No long addresses. So I doubt, that this is related to bug 42295. I have attached a ZCO log.