Clear Windows Credentials

This is a possible solution to Outlook not accepting an app password and asking for it in a loop.

1. Close Outlook

2. Click the Start Menu (or press the Windows key), and then search for Credential Manager.

3. Click on Credential Manager to open it, and then click Windows Credentials.

Under the Generic Credentials heading, find the credentials that have Microsoft Office(15/16) and your email address.
For example: MicrosoftOutlook16_Data:SSPI:example@yourdomainexample.com It may not be exactly as shown, but use it as a base guideline.

4. Click the arrow next to the entry to display it.

5. On the detail page of the credential entry, click remove to remove the credential from the manager.

6. Repeat this process for all credentials matching Outlook(15/16) and your email address. There may be anywhere from one to three entries listed. Also include the credential for outlook.office.com.

7.Close all windows, and re-open Outlook. When Outlook prompts for your credentials, enter the full email address and app password for the mailbox, and check the box Remember My Credentials.

8. If this process is successful, your Inbox should display, and mail should start syncing.

From: https://support.rackspace.com/how-to/clear-credential-manager/