Recently had a client with Office 365 trying to Cache a Shared Mailbox. Outlook brought up a load of Sync Errors

Your server administrator has limited the number of items you can open simultaneously. Try closing messages you have opened or removing attachments and images from unsent messages you are composing.

If the server is self hosted you can remove the restriction via  : http://henrysluiman.blogspot.com.au/2013/02/how-to-increase-number-of-open-items.html

Microsoft fix for this is removed the Cache! https://support.microsoft.com/en-us/help/3115602/performance-problems-when-you-try-to-access-folders-in-a-secondary-mai

After rebuilding the OST and Profile and still no luck I started Outlook in Safemode which enabled the downloading of the emails

Once downloaded I could start Outlook Normally and all worked fine