Using older versions of Outlook for Mac in business environments
I've finally found a workaround that works for me. NOTIFICATIONS. Specifically, Notifications in Apple's System Settings.
Do these steps:
BEFORE you launch Microsoft Outlook, go to your Mac's System Settings, click on Notifications, scroll down to Microsoft Outlook, turn off "Allow notifications" by setting the top right button to the off position, quit System Settings, launch Microsoft Outlook (it should now open without a problem), go back to your Mac's System Settings, click on Notifications, scroll down to Microsoft Outlook, turn on "Allow notifications" by setting the top right button to the on position, then quit System Settings. You'll have to do these steps EVERY TIME you want to launch Outlook. (Or, if you don't need the Mac to alert you when you get notifications, you could just leave the "Allow notifications" off, and not have to worry about going into System Settings and doing these steps every day.)
Over the last few days, I've done these steps on both of my M1 MacBook Pros in my office that have this issue, and it's been a successful workaround.
Like many others on this thread, my problem started on or around March 19, so Microsoft did something on that day, or an Apple Sonoma update broke some Microsoft programming. And, it's seemingly got something to do with Notifications at the system level.
So, Microsoft (or Apple) needs to figure out a fix.