Greeting!
Symptoms
You run Microsoft Outlook 2016, and when
you open Mail in Control Panel and then
click Add on the Data File tab, you may
receive this error message:
An unknown error occurred, error code: 0x8000ffff
Workaround
To work around this issue, add the data
file in Account Settings in Outlook 2016 by following these
steps.
Regards
Nebiyuy