309 votesAndrew Glennerster commented
I had this problem (Outlook 16.22 for Mac) and asked Outlook Support. They pointed me to this:
Running this (with Outlook closed) then restarting my Mac and Outlook fixed it.
1. Quit and re-launch Outlook.
2. Open this link -> https://github.com/pbowden-msft/FlightReset
3. Click "Clone or Download" -> select "Download Zip"
4. Open the Zip file
5. Right-click on "FlightReset" and then 'Open with Terminal'.