Sign-in loops in Classic Outlook can be caused by a few different background issues; so, to better understand your situation, could you please share some information:
Can you access your email on the web? Please try logging into outlook.office.com via a web browser. Does it let you in, or do you get an error there as well?
Where exactly is it telling you to sign in? Are you seeing a small pop-up box continuously asking for your password, a yellow banner at the top of Outlook, or does it say "Need Password" in the very bottom-right corner of the Outlook window? If possible, kindly share a screenshot of the error message.
Have there been any recent changes? Did you recently update your Windows password, or did your company's IT department recently require you to set up Multi-Factor Authentication (like the Microsoft Authenticator app)?
I look forward to your response.
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