Problem: When using Microsoft 365 with Office 365 Outlook, Outlook continuously is prompting for the password when adding a new Outlook profile or suddenly occurs when using an existing Outlook profile.

Cause: Microsoft 365 requires Modern Authentication for devices running Windows

Solution: Modify the Office 365 Outlook Registry Keys used for Microsoft 365

1. Open the Registry Editor (regedit) and navigate to the following location:

HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Outlook\AutoDiscover

Delete the DWORD key ExcludeHttpsAutoDiscoveryDomain (Ensures Autodiscover is found when using https)
Delete the DWORD key ExcludeExplicitO365Endpoint (Note: This is only when using Microsoft 365)

Add or set the DWORD ExcludeHttpsRootDomain to a 1 (Excludes using the root domain eg. computerbusiness.com)

2. Navigate to HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\Identity

Delete the DWORD key EnableADAL
Delete the DWORD key DisableADALatopWAMOverride

Add or set the DWORD key Version to a 1

3. Navigate to HKEY_CURRENT_USER\Software\Microsoft\Exchange

Add or set the DWORD key AlwaysUseMSOAuthForAutoDiscover to a 1