What's odd is that I've seen the same behavior on Windows and Mac clients and it has happened on O365 tenants where MFA is enabled and ones where it is not enabled. Migrate the user to the new profile, no further issues.
The solution: Signing into a new user profile (doesn't matter if local or domain) on the same PC and signing in, successfully signs into O365. Reviewing event viewer, usually do not find anything other than event 0 from the Office 2016 Licensing service, which should be resolved by reinstalling Office but does not. Disabling IPv6 (found a suggestion on a Spiceworks thread somewhere), no change. Testing connectivity from on the affected system, no observable issues. Removing Office, rebooting, reinstalling from the user's O365 portal installer, no change. If it finds errors it claims to have "corrected" them but doesn't (no surprise there). Running Microsoft's Support and Recovery Assistant, with no change. Removing the Outlook profile and re-adding it, no change. Removed O365 credentials from Credential Manager, with no change in behavior. Testing OWA with the credentials successfully. When attempting to resolve on Windows 10, I have tried: Most of the time the typical resolutions mentioned below help resolve the O365 authentication issues but not always. I've witnessed this on both Windows and Mac clients. If the user inputs the wrong password, the dialog box will report that.
The symptom is that the user is prompted for their O365 password in Outlook, which they supply, the dialog box spins its gears, then requests the user's email or password again, ad infinitum. I work for an MSP and over the past few months, I've observed a recurring issue with Office 365, specifically with Outlook, causing a login loop. Hi r/sysadmin, longtime lurker, first-time poster.