I believe this is resolved now.
We too had the same issue with all our mac users and last week we discovered 'it resolved itself' despite MS asking us to recreate all local accounts (including personal machines), removing Office, Removing / Adding MX Records etc etc etc.
Post fix the support tech from Office for Mac advised ''I’ve been hearing that some changes seem to have been performed on the server. As agreed, I contacted my colleagues from the server team and it seems that the issue was caused by a server interruption in one of the data centers. The problem was corrected after proper datacenter background replication took place''
The server for Office 365 post 'upgrade' has infact been changed to 'outlook.office365.com' , so you may need to recreate the Outlook identity and ensuring that Outlook Auto Discover picks this up rather than the old server ''pod51xxx.outlook.com'