Thanks for the help, now I can log in. Two more questions:
1. At the first login attempt a browser window comes up (as expected) and I log in the usual way using shibboleth. Now I have to go back manually to the Moodle Mobile app (I understand that). But after going back to the app I had to add the moodle site again (type in again the site URL). At that point the app got me logged in. Is that normal behaviour, or I did something wrong? How can I avoid having to add the site twice?
2. In the setting of Plugins, Web services, External services, local_mobile there is a setting called: "Authorized users only". What setting should I use for this one, and what is it good for?