Auto-discovery services suggest URL: This error indicates that we were unable to connect to the Source system using the Source URL specified on your connector. We then queried Exchange auto-discover services, which responded with an suggested URL. This error message is asking you to check if the suggested URL is correct and, if so, to update your configuration.
Check if the suggested URL points to the Source. For example, if auto-discover settings already point to the Destination system, then the suggested URL will be incorrect as a Source URL, and so should not be used. Otherwise, try specifying the suggested URL on your connector as the Source URL and resubmit mailboxes for migration.