Using older versions of Outlook for Mac in business environments
The suggestion by Vladimir was about certificates.
I’ve seen this problem numerous times with corrupted Keychain entries.
Outlook stores the password in the Keychain. Launch the /Applications/Utilities/Keychain Access and look for the corresponding entry.
The easiest from there is to delete it (which will allow Outlook to recreate a clean new one next time).
You can also run the Keychain First Aid command from within the application to try and correct any other issue your keychain might have.
Corentin