I've been helping my dad with this issue which is not good for his health. 😢
Anyway, to summarize again,
1 - shut down outlook completely, and ensure the database daemon is not running as seen in the Activity Monitor.
2 - if the database daemon is running, then kill it with the activity monitor.
3 - run the terminal app and type the two commands, pressing the return key after each is typed:
defaults delete com.microsoft.Outlook (to delete the old preferences).
killall cfprefsd (to kill the cached prefs)
4. Launch Outlook
5. Never close the main window, only minimize it if you have to.
6. IF the main window is closed by accident then repeat from step one.
My dad had the issue of not having native folders show too, of which the above process addresses.
I suppose a reboot works as well, the database daemon only starts and stays started after the first time outlook is launched.
This has been working so far for the last week and a bit for my dad.
Let me know if any of this is not required for those of you who are experiencing this problem directly.
Please bug Microsoft lots about this!
Cheers