Ok, I've been looking into this and I found the issue with the QR code.
I was confused by the error message, but it isn't related to your admin settings. The QR login using authenticated QRs (the one that you find in the user profile) only works for sites with a Pro subscription, you can learn more about subscriptions in the Apps Portal.
If you don't plan to purchase a subscription, you have some options:
- Leave it as it is, after users click the OK in that modal, they will see a login screen where they'll be able to introduce their credentials.
- Give them an unauthenticated QR code, with that QR code they won't see that message (but they'll have to introduce the credentials manually anyways). You can obtain this QR in the Apps Portal with a free account (under App customisation > QR code).
- Disable logging in with a QR code and they won't see the QR code in their profiles (in your site administration under Mobile app > Mobile authentication > QR code access).
Sorry for the confusion, I hope it's clear now.