Outlook 2016 doesn't accept password or app password

Issue not resolved - more updates on my experience.

1. I wrapped up my international business trip and flew back to the US. After landing in the US, my phone immediately went into the password prompt loop. Everytime the phone prompts, it get a notification tone and it was a pain while waiting to pass through Custom with the phone beeping. It's the first time the phone seen T-Mobile 5G network in more than 2 weeks. It lasted over 1.5 hour and it's only when I am half-way home on a drive when the password prompt went away on its own.
2. On my home computer, I didn't get any prompt initially and 2 weeks of emails were downloaded into desktop Outlook app (from Office, not Window's new Outlook PWA). Then about 2 hours later, desktop Outlook 2013 started the password prompt look. I have modern auth enabled through regkey. My work computer Office 365 with latest updates also went into the password prompt loop.

This issue is FAR from being resolved. Prior to 1/23/2024, I almost never had this level of password prompts. At most when MS is messing with their infrastructure, I get a couple of hours of this issue once in a long long time....definitely not 2+ weeks like this time.

I haven't had any issue on 9Folders since I deleted all my MS accounts (making sure to back up settings first) and re-added them.

Thunderbird issues were sorted by changing the server name to outlook.office365.com and authentication method to OAuth2
Outlook still prompts for password but doesn't impede mail.

This issue has now reappeared, it worked for the last 2 days but now asks for the password again when logging in to outlook office 2016 professional plus

Another authentication error. 2 outlook.com accounts. Immediately took the sign-on credentials, but issue obviously not resolved. This was on 9Folders. Also, server authentication error on 3 outlook accounts on Thunderbird just now. Again, reset and retrieved and all worked. Nonetheless, the consistent failure of login credentials continues on MS servers.

Windows 10 22H2 on Surface Pro 3 running Outlook 2013 - same boat for the last two weeks

Was getting password prompting w/o 2FA or App password every few minutes. Would sign in 3 times, then cancel. Outlook would connect for a while, then prompt again. Rinse and repeat multiple times a day.

Been watching this thread and checked the Reddit page today. Saw this link and gave it a try: https://azvise.com/2022/08/23/enable-modern-authentication-for-outlook-2013/

Updated Regedit > Closed/re-opened Outlook

I know it's not "the" fix for MS's issue, but might help as a work around for the moment.

So far, in the last 3 hours, no prompts. Will update this again in another day.