Recently set up 4.5.6 on Centos 5 for some testing. Installed ZimbraOlkConnector-4.5.6_GA_1023_4.5.288.msi on XPsp2 and Outlook 2003.
While setting up some calendar shares, Outlook locked up. After I killed the process and started Outlook back up the - Open - 'Other User's Mailbox' tab disappeared. Uninstalled Office, reinstalled with no results.