Proxy configuration appears to be the most common issue when this error is encountered. Can you verify your settings if you use a proxy?
Alternatively, it could be an issue where the WSUS server is configured to use SSL but you're not specifying the needed port in the WSUS connection setup. That can be checked (and tested) in the Update Services Registration Management wizard, which is located in Update Services (in the Actions pane on the right).
One other (not directly related to this error) thing to consider: Since this is in a 'remote domain', make sure you have added a credential from the remote domain and assigned that to a related rule in your Credential Ring so that you can authenticate to that WSUS server (unless there is a full bi-directional domain trust in place).