Hi,
I wanted to share my experience with this problem, I'm facing the same error as you guys when I try to add my work email.
After the MS event recently, I gave the new outlook (preview) another shot by switching it on, in the old Outlook app and I saw they updated the icon and fixed some features that were buggy before. But I still couldn't log in to my work account and got the error message.
By the way, I have the business basic plan and I noticed on the MS 365 website that the basic plan doesn't include desktop apps, only web version. So, I tried adding my school email which is "Business Standard", and which has the desktop apps included and it worked fine, no error at all.
So, I upgraded one of my business licenses from basic to standard and now I can add it to the new outlook desktop app. So, it seems like, if you are already logged in to the old or new outlook apps, DON'T log out or you won't be able to log back in unless you upgrade. I wish Microsoft would have told us this clearly or maybe I missed it.