I have the same issue, however, this has only started to occur since reverting to the "old" style outlook.
Using the New Outlook style I do not get this issue (but at the loss of functionality!).
I rolled back as Microsoft allow rules to be sent on non-Microsoft emails (e.g. Gmail) however this "feature" is not available in the new version and the promotional clutter was filling my inbox.
I am currently searching for a way to resolve this - the pop-up notification accepts my credentials (as it fails if the password is input incorrectly), however despite ticking the "remember me" setting it does not remember me!
The cynic inside of me thinks Microsoft has done this to force you off the old version of Outlook and force you to migrate to a Microsoft address to make use of the features that you previously got used to but no longer work (e.g simple rules) in the new version.