Microsoft, just please effing MAKE THIS STOP!!!
It has been a constantly problem on BOTH my computers. Nothing in this thread has ever worked to solve it. I've nuked and reset the Outlook profile multiple times on both machines. I have ran with O365 mail accounts from 3 different domains in Outlook for YEARS and never had a problem until about the last year. Sometimes I can go a week without being prompted for credentials. Other times like today it will do it half a dozen times a day on EVERY account.
I have specifically had to BLOCK MFA from deploying on 2 business domains because of this constant issue. This bug is effectively resulting a massive security hole because it would be completely unusable with MFA active and the constant login prompts (I tried it for a week before I had to go in and disable it in a rage).
I'm seriously considering abandoning O365 and MS over this and the future of "new Outlook".