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. We can click the "more details" link to see more info about the error, but it tells us nothing specific. Currently, our fix to this has been to add the following registry entry:
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. This seems to be our most effective fix, and it's the one we arrived at after hours of forum digging and web searches. Information and fixes from Microsoft have been scant as well, outside of the registry key one.