I started experiencing this problem today. It appears to have been the result of an automated Windows Update (for Windows 10).
As many have already stated, the problem seems to be tied to New Outlook. If I revert back to Old Outlook, it works perfectly fine, but as soon as I try to enable New Outlook again, the problem re-appears.
I've been using New Outlook for over a year, so it's quite surprising to be affected out of the blue. Perhaps some Windows 11-specific security changes were retroactively pushed out for Windows 10 as well? It seems that most users in this thread are running Windows 11.
Another alternative that works for me is to use the browser e-mail services instead. So if you truly need to access your mailbox asap, this could be the way (if reverting back to Old Outlook doesn't cut it, that is).