Outlook 2016 doesn't accept password or app password

Not just the same password, but the same account, tied to your windows login or added to windows. In my case I've never seen any prompt or authentication to add the account to outlook (or at any point after), it just adds, pulling from the account info with the modern auth process that was validated when added to windows. I don't even sign into windows on my personal machines using MS account (I prefer a local account), I just added it to the additional accounts in windows. On work machines the login to the machine is a MS account (using our company's domain and custom modern auth screens) so that takes care of it.

From what I've noticed on my Gmail android, it isn't that sending an email or synching causes the issue, it is that the issue exists and that's what makes it realize it, it can't complete the action, and that's when the "credentials required" comes up. Or it also comes up on its own after a while even if I didn't try to do anything. Bouncing the network usually resolves it.

Today so far, multiple changes of networks and IPs, IPv4, IPv6, all working fine on Gmail.

Same password - I do not have or want app password because I am using modern auth across all my devices and have enabled 2FA on my accounts. These are done quite some time ago. It's true that during account add, the modern auth will open a browser session for me to login and perform 2FA/MFA challenge - and not showing the "password prompt." After the setup is completed and when this issue occurs, I am seeing the prompts. Again, it has been improved since the onset of 1/23 as I have noted in my comments. However, it would not call this fixed if many of us are still experiencing this issue, however sporadic.

As far as I can tell, changing of IP address will have high likelihood for triggering this issue. Also, if you initiate any creation or modification of your email, calendar event, or contacts, to which the client needs to sync with the server, this could also lead to the prompt.

Please let your developers know that the people with 2FA/MFA setup are also having the issue.

Based on experiments over the last few days, I think there are multitude of issues at play:

1. Possible authentication/security related issue on the server-side - it sounds like the team has improved or at least they thought they fixed the issue.
2. Code changes related to 1) has not propagated to all servers and it has been several days, so one would expect those code changes should have reach all the key servers.
3. Still possible issues related to IP address changes. I have noticed an increase in password prompts when I transition from 1 IP address to another. Scenario is:

a) leave office (one IP address) and transition to different IP on the mobile network - this surely will cause prompt(s)

b) enter hotel and transition from mobile network to another IP address of the hotel's - this will likely cause the prompt(s)

c) hotel WiFi may disconnect, so the device momentarily fallback to mobile network - this could cause prompts.

In my case, office IP address and hotel IP address are the same, so I do not see the security risk here. I also not introducing new devices onto my account, so the known/assigned UUID to the device should provide some level of assurance. Competing platform does not have such issue with their email accounts. And to be blunt, this seems like a design issue not to factor into consideration of these very likely network transitions.

In short, either the fix has not reached all servers or the fix still have weaknesses against real-world scenarios.

From not the community crew:
If there were the account difference which are not the same licence in the windows login and outlook login, and primary account or not, and something account related?
I found that the my 3 free outlook suffix accounts (created separately in Hotmail, live, outlook period and suffix. and lost the hotmail and live suffix.) in one profile in outlook 2021 behaves differently, while using outlook.exe /resetfoldernames or other command lines, even trying set each one as default store.
It was going to changing all the 3 accounts to the outlook language whether the default store account is, but became unpredictably several month ago, even change the default store.
I set one exchange account in one profiles now.

https://learn.microsoft.com/en-us/outlook/troubleshoot/authentication/outlook-shows-disconnected-after-enabling-modern-authenticationOutlook shows "Disconnected" after you enable modern authentication in Microsoft 365 Article 07/22/2022 Applies to: Outlook, Microsoft 365
A known issue in Office creates a miscommunication between Office and Windows that causes Windows to provide the default credential instead of the appropriate account credential that is required to access the mailbox.
Resolution The most effective solution to this issue is to re-create your Outlook profile..
More information
For Monthly Channel Microsoft 365 subscribers, the fix to prevent this issue from occurring is available in builds 16.0.11901.20216 and later.
For Semi-Annual Customers, the fix is included in builds 16.0.11328.20392 and later.

https://techcommunity.microsoft.com/t5/outlook-blog/how-licensing-works-for-work-and-school-accounts-in-the-new/ba-p/4047361Some
How licensing works for work and school accounts in the new Outlook for Windows By Allen Filush Published Feb 02 2024 12:28 PM
Some organizations choose to provide multiple email addresses with different licenses to their users. In the classic Outlook for Windows today, users could install and activate the desktop app with an initial account that was properly licensed for desktop use (e.g. Business Standard, Enterprise E3). Once Outlook was properly licensed by that initial account, users could add additional accounts like Business Basic and Enterprise E1 as secondary accounts.

If you purchased a Microsoft 365 Personal or Family subscription with a Microsoft Account that was created with a non-Microsoft email address, you will need to add an Outlook.com alias in order to connect it to Outlook.

Do you use the same login for your machine as you do for email? If so you should never see a password prompt, not even when adding the account to Outlook.

Even if your PC login is different, you can add the MS account associated with the email account to windows and it should also never ask for credentials in outlook.

Both my personal O365 and work O365 work this way, have not seen a password prompt in Outlook 365 (home or work) in years.

Maybe different accounts on different server farms are being impacted differently, this is just my experience. But if you've gotten password prompts even before this issue existed, that tells me something is messed up with your authentication config/hierarchy.