Outlook Client popup

This is the authentication popup. If you're saving docs straight to SharePoint Online this can sometimes happen and you have to auth several times. Usually it can be cleared by going into File > Office Account on any Office application and fully signing out of all accounts there and then re sign in with your Office account.

Looks like an authentication popup. Perhaps trying to authenticate with existing logged-on user credentials. Also, check if you've any SharePoint Lists added in your Outlook profile (like a team calendar) which might be trying to get authenticated.