If you are getting a prompt asking for you to sign into outlook, please enter your login details to your account when prompted. If the prompt appears each time you try to open Outlook (i.e. not a one off sign in) please raise a new case online and reference the title of this article.
In your case, please include the following:
Who this is affecting (is this happening for all users?
When did this start happening?
Name, email address and contact information of all affected users.
The specific message or image of the error if possible.
Once we have received your case, this will be actioned as soon as possible.