Using Classic Outlook on Windows for personal email, calendar, and contact management
This is a widely reported problem with Outlook at the moment caused by the recent update and Microsoft indicate they are working to patch this problem.
Close Outlook.
Open File Explorer.
Paste this into the address bar and press Enter.
%localappdata%\Microsoft
In the resulting folder, delete the Olk and OneAuth folders.
Restart Outlook.
You may need to recreate your account on Outlook after performing these steps.