I have ONE account that is not syncing on mobile devices, either Android or iPhone. Other accounts work fine on both devices. So by process of elimination, it is apparent that it's the specific account, not the app or the phones. Desktop OWA works just fine for this user. Server is onsite Exchange 2016. Mobile Outlook works for everyone else. Normally it doesn't throw any errors, just has empty inbox as if it was a brand new mailbox. Even when sending, it silently goes into the Sent folder but doesn't actually send.
A few days ago while troubleshooting, this error popped up: https://photos.app.goo.gl/m7UHbfMRsSkPWALi8
I tried to search for it but Google returns very limited results and they reference Office 365 login issues, which does not apply to me.
On the same phones where Outlook doesn't work for this user, a third party app Spark works just fine, sending and receiving.
If I don't find an easier way, I will recreate the account and mailbox, but that is a nuclear option I'm trying to avoid.