Current Status: issue – no issue
Figure 11: Instance-GUID issue in outlook 2003
The problem behavior can be triggered by networking issues, patch applications, server reboots, etc. It requires Outlook 2003 to lose connectivity to exchange and have a delegate mailbox logon attempted on a connection before the primary mailbox logon is attempted on that connection. This can affect users who are accessing shared mailboxes or folders of other users from their Outlook 2003 while connecting to exchange 2010.
Currently there is no proper resolution available for this issue. However, to remediate this issue temporarily, simply re-resolve the Exchange server name in the Outlook mail profile. If possible, you may also remove the shared mailboxes or shared folders (like calendars) from the profile, preventing this issue from occurring in the future or create a new mail profile in Outlook.
microsoft is working to investigate this issue for a fix. You can contact Microsoft Support to report this issue and request further assistance. Currently no KB article exists for this issue.