After enabling MFA for a customer they had problems signing in Outlook. It keeps prompting with the old style of logon instead of the modern authentication.

To resolve this we run the following command to enable modern authentication connections:
Set-OrganizationConfig -OAuth2ClientProfileEnabled $true

Users who were previously logged into Office 365 in their Outlook clients — even clients that support Modern Authentication — might still experience an issue where the Modern Authentication browser window does not appear. In many situations, this can be fixed by following the steps below to clear ADAL credentials from the Windows Credential Manager. Please note: If clearing the credential manager cache does not result in a web browser popup when logging into a Modern Authentication capable version of Outlook, deleting and re-creating the mail profile may be necessary.

Windows 7

1. Log into the Windows 7 computer as the user with the Outlook issue.
2. Navigate to the Control Panel.
3. Click User Accounts > Credential Manager.
4. Under “Generic Credentials” you will see items similar to “MicrosoftOffice16_Data:ADAL:adc5ee9c-…”.
5. Click on an item that has “ADAL” in the name. Click Remove from vault.  On the confirmation pop-up, click Yes.
   
6. Repeat Step 5 for all “ADAL” credentials.
7. Open Outlook and attempt to log in again.

Windows 8, 8.1, 10

1. Log into the Windows 8, 8.1, or 10 computer as the user with the Outlook issue.
2. Navigate to the Control Panel.
3. Click User Accounts > Credential Manager.
4. Select Windows Credentials.
5. Under Generic Credentials you will see items similar to “MicrosoftOffice16_Data:ADAL:adc5ee9c-…”.
6. Click on an item that has “ADAL” in the name. Click Remove. On the confirmation pop-up, click Yes.
   
7. Repeat Step 6 for all “ADAL” credentials.
8. Open Outlook and attempt to log in again.