How to reproduce:

1. Create a course with groups.
2. Enrol a user that doesn't belong to any group.
3. Create a Chat activity configured to use Separate groups.
4. In browser (LMS), open the chat and check that you can open the chat and view past sessions (if any).
5. In the app, check that trying to enter the chat or viewing past sessions throw an error saying that you don't belong to any group.

The WebServices that are failing are these ones:

mod_chat_get_sessions
mod_chat_login_user

The app sends groupid=0 in both cases, and the WebServices return the error "notingroup".

IMO it should be consistent, either fail in both cases or work in both cases.