Graph API: access to /drive/root for one user always returns Access Denied

Accessing other users' /drive/root and /drive/root/children works fine.

But for this specific user it returns:

"error": {
        "code": "-2147024891, System.UnauthorizedAccessException",
        "message": "Access denied. You do not have permission to perform this action or access this resource.",
        "innerError": {
            "request-id": "4214d5c3-0753-4df5-bc6d-1b23e221804c",
            "date": "2016-01-21T20:44:30"
        }
    }

The user had troubles logging in and had to reset his password (administrator initiated the reset password process for him). Now he can login, but after that we started getting those errors.

Please help.