No notes are displayed OR message "no roles defined in admin->appearance->graderoles".

You can fix this error by going to /grade/lib.php and change the following line ( line 3682 in lib.php v1.3):
$configvar = get_config('', 'gradebookroles');
to
$configvar->value = get_config('', 'gradebookroles');

See : http://moodle.org/mod/forum/discuss.php?d=62380

Thanks to Chris Meyer