Can't connect to Microsoft Current User Connector

We can’t connect to the Microsoft Current User Connector so that users can send emails from their sign-in account.

The email address on the Email Accounts List Page is always empty in Version 24 (on-prem).

This worked perfectly in Version 18 (on-prem), shows the user’s email address and allows us to send a successful test message.

We have tried using the identical Client ID and Secret Value without success.
We have added the new Redirect URL and have ensured that ‘Allow HttpClient Requests’ has been enabled on the extension.

Would anyone have any idea why this isn’t cooperating in Version 24?

Thanks in advance.