this error appears when an error is returned by the token.php script. Actually we should add some kind of debug mode on the app to help admin to test the app. Many different errors are returned to the app and the app just display one unique error message.
You could check all the following points, it is most likely something going wrong there: http://docs.moodle.org/22/en/Web_services_FAQ#What_is_the_.27Access_control_exception.27_error
Note that many of these points should be valid when you enable the mobile service in the admin.
Moodle for mobile
moodle on iPhone app - Error
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