I came across this thread: http://community.office365.com/en-us/f/153/p/238265/751816.aspx#751816
I am encountering the same problem using Microsoft Outlook for Mac Version 15.6 (150113) and an Exchange mailbox hosted on Office 365.
The calendar was imported to the Exchange mailbox from a Windows PC using Outlook and upon downloading the calendar items to the Outlook for Mac mailbox, every imported item is triggering the same error, subsequently he is having sync issues between the calendars on his two Mac computers which i suspect this is likely related to. Any word on how to fix this automatically?