After you enable modern authentication in an Office 365 tenant, Microsoft Outlook cannot connect to a mailbox if the user’s primary Windows account is a Microsoft 365 account that does not match the account they use to log in to the mailbox. The mailbox shows “Disconnected” in the status bar.
A known issue in Office creates a miscommunication between Office and Windows that causes Windows to provide the default credential instead of the appropriate account credential that is required to access the mailbox.
This issue most commonly occurs if more than one mailbox is added to the Outlook profile, and at least one of these mailboxes uses a login account that is not the same as the user’s Windows login.
For Monthly Channel Office 365 subscribers, the fix to prevent this issue from occurring is available in builds 16.0.11901.20216 and later.
For Semi-Annual Customers, the fix is included in builds 16.0.11328.20392 and later.
For devices that are already affected by this issue, refer to the Resolution section.