To fix this pop-up window appearing everytime you open Outlook, we need to remove your cached credentials for Outlook from the credential manager. Please follow the steps below.
1. Close Outlook.
2. In the search bar on the Start Menu, type “cmd” no quotes, then press enter, should look like this:
3. Highlight this line below and press Ctrl + C keys at the same time to copy it.
4. Right click in the black box and choose "paste" then press Enter.
5. 1. Highlight the line that says “MS.Outlook.15:yourname” and click the remove button.
6. Click close, then log off the computer, log back in, and reopen Outlook.