The OAuth process launched by the app does not complete (the user is able to log in but it is not redirected back to the app) when new policies requiring agreement are added to the site. This could also happen the first time they access to the site if there is a mandatory license.

I've reproduced this with new users but I think it would happen the same for existing policies when a new mandatory policy is added to the site.

There is a more detailed description here: MOBILE-4078

Reproduction steps
On the Moodle site (LMS) as a site administrator:

1. Add a site policy (Site administration > Users > Privacy and policies > Policy settings). It can be either 'Default (core)' or 'Policies (tool_policy)' as it happens on both.
2. Configure Google OAuth
3. Create a user account with OAuth2 authentication method, use a Google account email. (You need to be able to log in using that account.)

On a mobile device:

1. Open a browser and log in on that Google account
2. Open the the Moodle app and add the Moodle site (LMS)'s URL
3. Tap on the Google authentication
4. (If you have more than one Google account you will need to select first which want you want to use)
5. Accepts the pending policy(ies) in the browser
6. You will see an error message and you will NOT be redirected back to the app
   

Workaround: Change the Type of login (tool_mobile | typeoflogin). Site administration > Mobile app > Mobile authentication, to via a browser window or via an embedded browser. (For more information please visit the Moodle app guide for admins - Mobile authentication: SSO authentication method.)