Has anyone else come across this, or found a permanent solution (short of turning MFA off) ?

There doesn't appear to be any pattern to the issue occurring or the problem being limited to any specific computer or user. In each case the fix has either been one of the above items or a combination of them. I have had different experiences with a fix ranging from.

and so we repeat the same process again, and again. Then a couple of seconds later the modern authentication screen appears again asking for your password. You approve the sign in on your phone then the authentication window on the computer closes. Please respond to continue' (I am using the Authentication App but the same problem occurs if you are using SMS). You enter the correct password then click 'Sign in', You are then displayed with 'We've sent a notification to your mobile device. Has anyone else experienced Outlook 2016 stuck in an authentication loop when you have multi-factor authentication (MFA) enabled on Office365? The sequence is that Outlook displays the modern authentication screen for you to enter your password.