Outlook 2016 doesn't accept password or app password

Yes. I firmly believe the intermittent nature itself of these issues both with prompts and slowness is due to the extreme throttling they are applying to the servers. My experience now using IMAP and Thunderbird is with intermittent but consistent slow responses from servers when opening and closing folders, and downloading and marking messages. At times responses are normal and unremarkable, while at other times I get very slow responses from the servers.

As a side note about 1-2 times a year for about 5+ years, email would get the authentication error that would last 2-3 days. Web outlook would be fine so confirmed never a password issue.

I concur! I have the exact same experience!! It pops up on my desktop Outlook but more frequent on my Android device via its Exchange accounts setup. And as exactly as you said, it last a few hours or sporadically up to a couple of days and it would go away. I suspect because the issue went away in a short amount of time, nobody bothered to create a big thread to discuss it or it didn't get the visibility like this time.

For what it's worth, my desktop Outlook 2013 has been a lot better in the last 3-4 days. I get one-pop a day at the start of my day, like an overnight time-out with the authentication. I can simply cancel the pop and I am good for the entire day. My phone, however, experience more of the pops, but it does recover on its own, whereas over last weekend, it was out the entire few days.

As a few others mentioned, sync of email/contact/calendar are extremely important for most people in today's world and if MS does not find a way address this issue, then it's going to cause people to move away to other platforms. The key issue here is that at Outlook for Android app's sync'ed data is stored locally within the app or its cache, and it doesn't seem to be stored in the device-wide email/contact/calendar storage. Android or iOS's native Exchange support do that and store those in the device-wide storage. The benefit of that is other applications can access these device-wide storage, so you can for example access your contact list from a VoIP/chat app without having that app login into your MS or Google accounts.

11:28 AM CST. Authentication error. Could not sign in. Outlook.com account using eas server and 9Folders email client. Obviously still not fixed.

One thing I'm getting from this whole saga is that proprietary protocols are bad. IMAP, POP3, SMTP, CalDAV and CardDAV are all well supported open protocols, and if every email server made them available, the amount of breakage within email clients would surely be lessened.

Agree. Some guidance from MS on personal accounts vs. business accounts, and OAUTH, along with clarification on all this server "noise" would be helpful. Synching more than just email is very important. Calendar, contacts, tasks, notes etc. are important.