Deleting the main domain has solved the SSO portal issue.
However now apps that rely on the SSO can’t log in :
And the user interface gives me this error message : There is no app to list here, either because no web app yet is installed on the server, or because you don't have access to any. Please check with the admins of the server for more infos! This is easily solved in the default domain options under show other domain's apps (as stated here)