************ SOLUTION ********************
IM WRITING IN CAPS IN HOPES MORE PEOPLE GET TO SEE THIS REPLY
"in Chrome browser (my usual browser) 3 dots at top right -> Delete browsing data -> Basic (Time range: I changed from the "Last hour" to "All Time" -> Delete data. For good measure, I repeated the step for all open browser windows, even those not at Outlook page.
Then I closed all browsers, opened a fresh Chrome browser, went to Outlook page to sign-in and voila, issue was fixed and I could log-in as per normal again!"
THE SUGGESTION ABOVE WAS MADE IN THIS THREAD BY A USER NAMED Jellybeanie (thanks Jelly!)
This was the one solution that definitely and (so far) permanently resolved he issue.
TRY THIS OUT, GUYS