We are seeing some wierd issues during our Exchange 2010 migration. We have user's who are using Outlook 2003 with the encryption box checked in their settings.
Their primary mailbox is still on Exchange 2003 but they have 5 shared mailboxes that they have loaded in their Outlook profile that sit on Exchange 2010. They can open their mailbox fine but they can only expand one of those secondary mailboxes at a time in their Outlook session. If they need to expand one of the others they have to close out of Outlook and reopen and select one of the other secondary mailboxes. Then they won't be able to expand any of the other 4 secondary mailboxes.