1: Yes, because I imported the calendar entries into his exchange mailbox with my Windows Outlook client originally (this was months ago) and at the time of import confirmed that the calendar matched on OWA, additionally, it seems the entries all downloaded to his Mac Outlook clients in that they are visible and accurate, but every time you clear the cache on the calendar these errors fire again as the clients re-download the events from the server. I am assuming the fact that we are getting this error means that things will not be working as smoothly as they should moving forward with changes to the entries and as such may be related to the duplicates and missing items between the two Mac Outlook clients.
2: He has a Mac desktop and a Mac laptop, both were loaded with the new Mac Outlook 365 version right when it was announced the other month. At the same time we migrated from another Exchange host to Office 365, so i did the PST export / import on my Windows Outlook client from the old Exchange host to Office 365 Exchange, and then downloaded the mailboxes into fresh profiles on both the Mac computers. They both are connected to the Office 365 Exchange and a couple of secondary POP accounts.
3: The error does appear in both Outlooks. They have been running the same version as each other, but have been updated to the two releases you have made since the initial release. Current version is 15.6 (150113).
4: He is the only user on the account
5: His overall mailbox size is at least 5 GB and there isn't a PST currently, as they are exchange accounts, not POP/IMAP (unless you want the OST). If you want the OST, do you want it from one of the Mac computers or should i connect my windows computer and download the mailbox? Or, do you want me to export the calendar folder only from one of the mac computers or my windows computer?
