As per latest Winterfell:
```
[21-Aug-2018 02:52:22 Europe/Berlin] PHP Warning: Invalid argument supplied for foreach() in /usr/share/roundcubemail/plugins/kolab_addressbook/kolab_addressbook.php on line 231
```
Warning can be triggered when IMAP account has no address book and LDAP is unavailable (not quite common, but should be trivial to fix).