Single-sign-on fails

After starting of the app (v. 1.23 from F-Droid) it shows the button "NEXTCLOUD SINGLE SIGN ON (SSO)". Clicking the button shows the following message after a while: "Fehler Um den Nextcloud Single-Sign-On auf diesem Gerät zu nutzen, müssen Sie Zugriff auf Ihre Konten gewähren" "Error To use Nextcloud single sign-on on this device, you must grant access to your accounts"

The other Nextcloud apps, e.g. Deck, Notes, Nextcloud, work flawlessly on this device (Android 7.0). So either I'm incapable of granting access to my account (???) or there is a bug in this app.