This post shows you one of the possibilities to resolve the Outlook ProPlus issue. Outlook ProPlus cannot access to Office 365 email after migrating from Exchange 2007, this has been happening to many people. One of the ways to resolve this is. Open Control Panel.
Click on User Accounts.
Click Mail (Microsoft Outlook 2016) or something similar but not exactly the same.
Click on Email Accounts…
Remove the email account, reenter the similar one and reenter the password. After the close all the Windows and start your Outlook.