Azure AD Connect not authenticating during upgrade

Running an upgrade of the Azure AD Connect product to the latest version (2.3.20 - or something like that - latest version in July 2024) so I can migrate the service to a new server - but the upgrade fails on the "Connect to Azure AD" section.

Takes a little while to get to that as it is going through the motions of 'upgrading sync engine'. It then gets to the "Connect to Azure AD" where I provide the account for access.

The account is most definitely a global admin. I can also logon through a browser and complete the MFA process on the same server that I am getting this issue on.

I have tried to run the product via admin elevated cmd line with the /InteractiveAuth switch but the result is the same.

The failure is:

Unable to validate credentials due to an unexpected error. Restart Azure AD Connect with the /InteractiveAuth option to further diagnose this issue. (extendedMessage: An error occurred while sending the request. | The underlying connection was closed: An unexpected error occurred on a send. | Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host

WebException: The underlying connection was closed: An unexpected error occurred on a send. STS endpoint: HTTPS://LOGIN.MICROSOFTONLINE.COM/<MYDOMAIN>)

Any help would be greatly appreciated.

Thanks.

Jason