I got the following situation:
```$rcmail_config['address_book_type'] = '';
$rcmail_config['autocomplete_addressbooks'] = array('');```

At the moment this displays a error message on line 243 (Addressbook source () not found!) every time you type a character in the 'To:' field when composing a new message. This is very annoying. If there is a better way to fix this, I am very open to learn of it.