MS Outlook issue with Microsoft365 account on Mac
I do some adjunct instructing for a local university. They're a full-out MS shop. They give all students, staff, faculty, including adjuncts, Microsoft365 accounts. I had Outlook set up on Mac, Windows, and iPad for years. Then Outlook on Mac refused to connect to the server; it was always offline, and when set online by hand, it went offline again immediately. There were no problems on Windows or iPad. I set up the account in Apple Mail and in Thunderbird on the Mac; no problems. I had other accounts on Outlook on the Mac: gmail.com, Apple icloud.com, and MS outlook.com. All worked properly. I deleted the account that wasn't working from Outlook and tried to add it back; Outlook refuses to set up the account. It can't talk to the server.
This is very strange. It was working, and still works with 3rd-party email clients on that machine. Web access still works on that machine in Safari and Firefox. There are no problems elsewhere; I can even connect from an Ubuntu system using Thunderbird. I doubt that it's a server issue. It doesn't seem to be an issue with the Mac, not if other clients, including the web client, work. Outlook on the Mac simply refuses to access the server for that particular account and only that account.
Can anyone think of a reason why a MS client refuses to connect to a MS server?