Boss has shared their calendar to an assistant giving them manager rights.

Assistant can access calendar just fine in ZWC. But in Outlook (ZCO 6.5 or 6.6), whenever the assistant double-clicks on an event, they get

Cannot open this item. Cannot open the free/busy information. Cannot locate recurrence information for this appointment.
Boss uses ZWC and iCal. Also iPhone with ActiveSync.

I tried sharing the Boss calendar to another user, with same result. It seems there's something specific to the Boss's calendar that doesn't work when shared to someone using Outlook/ZCO.

Any ideas?