Outlook 2013 and 2016 client configured for UW O365 continues to receive Window Security sign in window requesting user authentication. The windows security message and sign in request is attempting to connect to another calendar or internet calendar, delete the calendar.
User is randomly presented with dialog windows as shown below because they have subscribed to a publicly shared calendar that is attempting to authenticate to Active Directory. Authentication is not possible or required, it is possible the subscribed calendar no longer exists or security is not allowing, or the link is corrupted.
Go to account settings, select the "Internet Calendars" tab, see if there are subscribed calendars and "remove" the calendar:
1. While Outlook is open, go to the "File" menu.
2. Select "Account Settings"
3. At the Account Settings, windows go to the "Internet Calendars" tab.
4. Review linked calendars and remove...