Outlook 2016 blank MFA screen for some users

Hi! Some things that can be worth checking and to test 

On users client:

• Does it help to clear credential manager on the device?
• Since you are on quite an old version of Outlook (1903). Confirm that the following Registry key exists. Without this, the Modern Authentication flow is not presented correctly. 
  • HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\Identity
    • EnableADAL = 1  (DWORD)
    • Version = 1  (DWORD)
• If possible, try and update a pilot users Outlook to a more modern version 

On the ADFS server config: 

• is "MSOFFICE" added as a WIA user-agent? This can cause issues if the user when sitting externally is on a VPN for example.

If its not like this for everyone when working outside the corporate network, my best bet would be either stale credentials in credential manager or that the REgistry key for Modern Authentication is missing on those specific users. 

I would also highly advice you to upgrade Outlook / Office from build 1903 since this was released in 2019. A lot of these modern auth related bugs have been fixed in later builds 

If this helps you feel free to mark my reply as a solution. Or let me know If I can assist further

Kind Regards

Oliwer