I just completed a migration from 5.5 to Exchange 2000 using NETIQ Migrator. Quite a few of my users now can't open OOA. Some of these users had the assistant enabled during the migration. They get an error: The OOA could not be displayed. A required action was not successful due to an unspecified error.
I have tried repairing and reloading Outlook XP, It's fully patched with SP2. My Exchange server has SP3.