Hello, MSLobao-
It was resolved in our case. The solution is: add 'My appointment' and 'My recurring appointment' back to <User Filters> in <Synchronization Settings for Outlook or Exchange>. You can access <User Filters> either thru Outlook
or CRM web. In our case, someone deleted all user filters completely. That causes when an appointment is launched in Outlook, Set Regarding does not shown in 'CRM' ribbon.
How to access <User Filters>: <Set Personal Options> => Synchronization tab => filters => <User filters>. Hope this helps you too.