Thanks for replying. I've installed the Moodle Mobile additional features plugin and now users could get the login screen, but they couldn't go in thru app though. This time there was a warning about unexpected redirection.
So I doubled checked my settings because I use Joomdle. It seemed to me that the joomdle auth wasn't compatible with the latest moodle mobile app, but it turned out that changing the conection method in joomla from "file_get_contents" to "cURL" with redirectless SSO solved the issue, and saved my day.