I haven't opened a ticket with MS yet but I may if my search does not prove fruitful. Speaking of research, I haven't found much info newer than December 2018. Additionally, we're looking to roll out Duo 2FA to all end users within a year, and while current research shows that the 2FA will continue to work as intended with ADAL/modern authentication disabled, I'm worried that it may break down the line.

My main concern is that these dialog boxes may return, prompting further user frustration and possibly generating tickets. It only affects users who have been migrated. We haven't seen this issue arise when a user is on a machine that was originally imaged with Office 365. Information and fixes from Microsoft have been scant as well, outside of the registry key one. This seems to be our most effective fix, and it's the one we arrived at after hours of forum digging and web searches. Once the key is added, and the user restarts Outlook, they receive a legacy authentication dialog box, enter their domain password, and connect to their mailbox without issue.

Currently, our fix to this has been to add the following registry entry: We can click the 'more details' link to see more info about the error, but it tells us nothing specific. We've been experiencing an issue with our Outlook clients after a user is upgraded from Office 2016 Standalone to Office 365 Business: After the click-to-run installer is complete, and often after a reboot, Outlook will show the splash screen when launched, then a mini-browser window will be displaying showing the dreaded 'An error occurred' message.

O365 modern authentication