When attempting to configure the Exchange credentials, the following error occurs in the GUI: "The server is unavailable". You have the option to Retry, Work Offline, or Cancel within the GUI error.
Outlook client is configured to use HTTP to connect to Exchange, but the Migration Manager Exchange settings are not.
Configure Migration Manager to also use HTTP to connect to Exchange. You will need to select "Connect Using Outlook Anywhere" and then fill in the Proxy Settings as they are configured in the Outlook client.