Параметр error_reporting управляет тем, какие типы ошибок, предупреждений и уведомлений будут отображаться при выполнении скрипта PHP. Он задаётся числом, которое представляет собой битовую маску, где каждый бит соответствует определённому уровню отчётности.

Ошибка «ERROR: Wrong 'error_reporting' option value and it wasn't possible to set it to required value (30711)» — баг Roundcube, он описан на Github в этой ветке комментариев.

Чтобы решить проблему, вам нужно прописать значение, указанное в ошибке, в файл index.php Roundcube и убедиться, что оно правильно сформировано и допустимо (при этом Roundcube заработает).

Сперва откроем наш файл:

cd /usr/share/roundcubemail/
nano index.php

Ищем сверху:

<?php
/**
 +-------------------------------------------------------------------------+
 | Roundcube Webmail IMAP Client                                           |
 | Version 1.4.15                                                          |
 |                                                                         |
 | Copyright (C) The Roundcube Dev Team                                    |
 |                                                                         |
 | This program is free software: you can redistribute it and/or modify    |
 | it under the terms of the GNU General Public License (with exceptions   |
 | for skins & plugins) as published by the Free Software Foundation,      |
 | either version 3 of the License, or (at your option) any later version. |
 |                                                                         |
 | This file forms part of the Roundcube Webmail Software for which the    |
 | following exception is added: Plugins and Skins which merely make       |
 | function calls to the Roundcube Webmail Software, and for that purpose  |
 | include it by reference shall not be considered modifications of        |
 | the software.                                                           |
 |                                                                         |
 | If you wish to use this file in another project or create a modified    |
 | version that will not be part of the Roundcube Webmail Software, you    |
 | may remove the exception above and use this source code under the       |
 | original version of the license.                                        |
 |                                                                         |
 | This program is distributed in the hope that it will be useful,         |
 | but WITHOUT ANY WARRANTY; without even the implied warranty of          |
 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the            |
 | GNU General Public License for more details.                            |
 |                                                                         |
 | You should have received a copy of the GNU General Public License       |
 | along with this program.  If not, see http://www.gnu.org/licenses/.     |
 |                                                                         |
 +-------------------------------------------------------------------------+
 | Author: Thomas Bruederli <roundcube@gmail.com>                          |
 | Author: Aleksander Machniak <alec@alec.pl>                              |
 +-------------------------------------------------------------------------+
*/

// include environment
require_once 'program/include/iniset.php';

// init application, start session, init output class, etc.
$RCMAIL = rcmail::get_instance(0, $GLOBALS['env']);

Меняем на:

<?php
/**
 +-------------------------------------------------------------------------+
 | Roundcube Webmail IMAP Client                                           |
 | Version 1.4.15                                                          |
 |                                                                         |
 | Copyright (C) The Roundcube Dev Team                                    |
 |                                                                         |
 | This program is free software: you can redistribute it and/or modify    |
 | it under the terms of the GNU General Public License (with exceptions   |
 | for skins & plugins) as published by the Free Software Foundation,      |
 | either version 3 of the License, or (at your option) any later version. |
 |                                                                         |
 | This file forms part of the Roundcube Webmail Software for which the    |
 | following exception is added: Plugins and Skins which merely make       |
 | function calls to the Roundcube Webmail Software, and for that purpose  |
 | include it by reference shall not be considered modifications of        |
 | the software.                                                           |
 |                                                                         |
 | If you wish to use this file in another project or create a modified    |
 | version that will not be part of the Roundcube Webmail Software, you    |
 | may remove the exception above and use this source code under the       |
 | original version of the license.                                        |
 |                                                                         |
 | This program is distributed in the hope that it will be useful,         |
 | but WITHOUT ANY WARRANTY; without even the implied warranty of          |
 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the            |
 | GNU General Public License for more details.                            |
 |                                                                         |
 | You should have received a copy of the GNU General Public License       |
 | along with this program.  If not, see http://www.gnu.org/licenses/.     |
 |                                                                         |
 +-------------------------------------------------------------------------+
 | Author: Thomas Bruederli <roundcube@gmail.com>                          |
 | Author: Aleksander Machniak <alec@alec.pl>                              |
 +-------------------------------------------------------------------------+
*/

error_reporting(30711);

// include environment
require_once 'program/include/iniset.php';

// init application, start session, init output class, etc.
$RCMAIL = rcmail::get_instance(0, $GLOBALS['env']);

Можно еще в .htaccess сделать так:

nano .htaccess

Ищем:

# This is a sample with suggested security and performance options

<IfModule mod_rewrite.c>
Options +SymLinksIfOwnerMatch
RewriteEngine On
RewriteRule ^favicon\.ico$ skins/elastic/images/favicon.ico

Меняем на:

# This is a sample with suggested security and performance options

error_reporting(30711);

<IfModule mod_rewrite.c>
Options +SymLinksIfOwnerMatch
RewriteEngine On
RewriteRule ^favicon\.ico$ skins/elastic/images/favicon.ico

После этого по обновлению страницы (F5) должен заработать Roundcube.