Roundcube - Server 500 error when trying to open HTML emails.

Hello,
Apparently, Roundcube updated to a version not compatible with PHP 7.2.4.
I got a report today, that Roundcube was hanging when clicking on listed emails.
After I click on any email to open it, a Server 500 error shows.
After investigation, I isolated that the issue is only with HTML-based emails. Raw text are no issue, and also emails are being served correctly via IMAP.
I did some research and the error appears to be connected to Roundcube requiring now a more recent version of PHP to operate. I disabled HTML in the meantime as a workaround.
Would this require a downgrade for Roundcube? and, how should I go about doing that safely? I see that upgrading the default PHP version wrecks things off. Could you please help me? My log is attached.

Code:

[07-Apr-2026 13:57:47 America/Chicago] PHP Parse error:  syntax error, unexpected 'private' (T_PRIVATE) in /usr/share/roundcube/program/lib/Roundcube/rcube_washtml.php on line 487
[07-Apr-2026 13:58:30 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (GET /webmail/?_task=mail&_caps=pdf%3D1%2Cflash%3D0%2Ctiff%3D0%2Cwebp%3D1&_uid=826&_mbox=INBOX&_framed=1&_action=preview)
[07-Apr-2026 13:58:30 America/Chicago] PHP Parse error:  syntax error, unexpected 'private' (T_PRIVATE) in /usr/share/roundcube/program/lib/Roundcube/rcube_washtml.php on line 487
[07-Apr-2026 13:58:32 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (GET /webmail/?_task=mail&_mbox=INBOX)
[07-Apr-2026 13:58:32 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (GET /webmail/?_task=mail&_action=list&_refresh=1&_mbox=INBOX&_remote=1&_unlock=loading1775588312415&_=1775588312375)
[07-Apr-2026 13:58:33 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (GET /webmail/?_task=mail&_action=getunread&_page=1&_remote=1&_unlock=0&_=1775588312376)
[07-Apr-2026 13:58:35 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (GET /webmail/?_task=logout&_token=9LltbbByrCD6VUrgOvNFiCIwgbjC0yyp)
[07-Apr-2026 13:58:58 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (POST /webmail/?_task=login&_action=login)
[07-Apr-2026 13:58:58 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (GET /webmail/?_task=mail&_token=TOlp6bGgJBfDRijmltQB06dPrj1YE8Hf)
[07-Apr-2026 13:58:59 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (GET /webmail/?_task=mail&_action=list&_refresh=1&_mbox=INBOX&_remote=1&_unlock=loading1775588338621&_=1775588338595)
[07-Apr-2026 13:58:59 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (GET /webmail/?_task=mail&_action=getunread&_page=1&_remote=1&_unlock=0&_=1775588338596)
[07-Apr-2026 13:59:06 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (GET /webmail/?_task=mail&_caps=pdf%3D1%2Cflash%3D0%2Ctiff%3D0%2Cwebp%3D1&_uid=958&_mbox=INBOX&_framed=1&_action=preview)
[07-Apr-2026 13:59:06 America/Chicago] PHP Parse error:  syntax error, unexpected 'private' (T_PRIVATE) in /usr/share/roundcube/program/lib/Roundcube/rcube_washtml.php on line 487
[07-Apr-2026 14:00:42 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (GET /webmail/)
[07-Apr-2026 14:00:47 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (POST /webmail/?_task=login&_action=login)
[07-Apr-2026 14:00:47 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (GET /webmail/?_task=mail&_token=NaGaMZrnfFnbTa0aWEFh342CdXF5pbgs)
[07-Apr-2026 14:00:47 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (GET /webmail/?_task=mail&_action=list&_refresh=1&_mbox=INBOX&_remote=1&_unlock=loading1775588446993&_=1775588446956)
[07-Apr-2026 14:00:47 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (GET /webmail/?_task=mail&_action=getunread&_page=1&_remote=1&_unlock=0&_=1775588446957)
[07-Apr-2026 14:00:50 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (GET /webmail/?_task=mail&_caps=pdf%3D1%2Cflash%3D0%2Ctiff%3D0%2Cwebp%3D1&_uid=958&_mbox=INBOX&_framed=1&_action=preview)
[07-Apr-2026 14:00:50 America/Chicago] PHP Parse error:  syntax error, unexpected 'private' (T_PRIVATE) in /usr/share/roundcube/program/lib/Roundcube/rcube_washtml.php on line 487
[07-Apr-2026 14:09:07 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (GET /webmail/?_task=mail&_caps=pdf%3D1%2Cflash%3D0%2Ctiff%3D0%2Cwebp%3D1&_uid=957&_mbox=INBOX&_framed=1&_action=preview)
[07-Apr-2026 14:09:07 America/Chicago] PHP Parse error:  syntax error, unexpected 'private' (T_PRIVATE) in /usr/share/roundcube/program/lib/Roundcube/rcube_washtml.php on line 487
[07-Apr-2026 14:09:08 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (GET /webmail/?_task=mail&_caps=pdf%3D1%2Cflash%3D0%2Ctiff%3D0%2Cwebp%3D1&_uid=957&_mbox=INBOX&_action=show)
[07-Apr-2026 14:09:08 America/Chicago] PHP Parse error:  syntax error, unexpected 'private' (T_PRIVATE) in /usr/share/roundcube/program/lib/Roundcube/rcube_washtml.php on line 487
[07-Apr-2026 14:09:12 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (GET /webmail/?_task=mail&_caps=pdf%3D1%2Cflash%3D0%2Ctiff%3D0%2Cwebp%3D1&_uid=957&_mbox=INBOX&_action=show)
[07-Apr-2026 14:09:12 America/Chicago] PHP Parse error:  syntax error, unexpected 'private' (T_PRIVATE) in /usr/share/roundcube/program/lib/Roundcube/rcube_washtml.php on line 487
[07-Apr-2026 14:09:16 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (GET /webmail/?_task=mail&_mbox=INBOX)
[07-Apr-2026 14:09:17 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (GET /webmail/?_task=mail&_caps=pdf%3D1%2Cflash%3D0%2Ctiff%3D0%2Cwebp%3D1&_uid=957&_mbox=INBOX&_framed=1&_action=preview)
[07-Apr-2026 14:09:17 America/Chicago] PHP Parse error:  syntax error, unexpected 'private' (T_PRIVATE) in /usr/share/roundcube/program/lib/Roundcube/rcube_washtml.php on line 487
[07-Apr-2026 14:09:17 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (GET /webmail/?_task=mail&_action=getunread&_page=1&_remote=1&_unlock=0&_=1775588956583)
[07-Apr-2026 14:09:17 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (GET /webmail/?_task=mail&_action=list&_refresh=1&_mbox=INBOX&_remote=1&_unlock=loading1775588956710&_=1775588956582)
[07-Apr-2026 14:10:18 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (POST /webmail/?_task=mail&_action=refresh)
[07-Apr-2026 14:11:18 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (POST /webmail/?_task=mail&_action=refresh)
[07-Apr-2026 14:12:18 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (POST /webmail/?_task=mail&_action=refresh)
[07-Apr-2026 14:13:18 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (POST /webmail/?_task=mail&_action=refresh)
[07-Apr-2026 14:27:58 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (POST /webmail/?_task=mail&_action=refresh)
[07-Apr-2026 14:28:58 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (POST /webmail/?_task=mail&_action=refresh)
[07-Apr-2026 14:29:58 -0500]: PHP Error: Failed to load plugin file /var/lib/roundcube/plugins/xskin/xskin.php in /usr/share/roundcube/program/lib/Roundcube/rcube_plugin_api.php on line 181 (POST /webmail/?_task=mail&_action=refresh)
[07-Apr-2026 14:30:34 America/Chicago] PHP Parse error:  syntax error, unexpected 'private' (T_PRIVATE) in /usr/share/roundcube/program/lib/Roundcube/rcube_washtml.php on line 487
[07-Apr-2026 14:32:25 America/Chicago] PHP Parse error:  syntax error, unexpected 'private' (T_PRIVATE) in /usr/share/roundcube/program/lib/Roundcube/rcube_washtml.php on line 487
[07-Apr-2026 14:34:24 America/Chicago] PHP Parse error:  syntax error, unexpected 'private' (T_PRIVATE) in /usr/share/roundcube/program/lib/Roundcube/rcube_washtml.php on line 487
[07-Apr-2026 14:35:56 America/Chicago] PHP Parse error:  syntax error, unexpected 'private' (T_PRIVATE) in /usr/share/roundcube/program/lib/Roundcube/rcube_washtml.php on line 487
[07-Apr-2026 14:43:06 America/Chicago] PHP Parse error:  syntax error, unexpected 'private' (T_PRIVATE) in /usr/share/roundcube/program/lib/Roundcube/rcube_washtml.php on line 487
[07-Apr-2026 14:43:08 America/Chicago] PHP Parse error:  syntax error, unexpected 'private' (T_PRIVATE) in /usr/share/roundcube/program/lib/Roundcube/rcube_washtml.php on line 487
[07-Apr-2026 14:43:12 America/Chicago] PHP Parse error:  syntax error, unexpected 'private' (T_PRIVATE) in /usr/share/roundcube/program/lib/Roundcube/rcube_washtml.php on line 487
[07-Apr-2026 14:44:39 America/Chicago] PHP Parse error:  syntax error, unexpected 'private' (T_PRIVATE) in /usr/share/roundcube/program/lib/Roundcube/rcube_washtml.php on line 487

Code:

##### SERVER #####
IP-address (as per hostname): ***.***.***.***
[WARN] could not determine server's ip address by ifconfig
[INFO] OS version is Ubuntu 18.04.6 LTS

[INFO] uptime:  19:01:48 up  5:15,  5 users,  load average: 0.08, 0.10, 0.09

[INFO] memory:
              total        used        free      shared  buff/cache   available
Mem:           3.8G        2.3G        249M        552M        1.2G        672M
Swap:          3.0G        347M        2.7G

[INFO] ISPConfig is installed.

##### ISPCONFIG #####
ISPConfig version is 3.3.1p1


##### VERSION CHECK #####

[INFO] php (cli) version is 7.2.24-0ubuntu***.***.***.***+esm12
[INFO] php-cgi (used for cgi php in default vhost!) is version 7.2.24

##### PORT CHECK #####

[WARN] Port 22 (SSH server) seems NOT to be listening

##### MAIL SERVER CHECK #####


##### RUNNING SERVER PROCESSES #####

[INFO] I found the following web server(s):
        Apache 2 (PID 840)
[INFO] I found the following mail server(s):
        Postfix (PID 24566)
[INFO] I found the following pop3 server(s):
        Dovecot (PID 24671)
[INFO] I found the following imap server(s):
        Dovecot (PID 24671)
[INFO] I found the following ftp server(s):
        PureFTP (PID 24782)

##### LISTENING PORTS #####
(only           ()
Local           (Address)
[anywhere]:993          (24671/dovecot)
[anywhere]:995          (24671/dovecot)
[localhost]:11332               (24620/rspamd:)
[localhost]:11333               (24620/rspamd:)
[localhost]:11334               (24620/rspamd:)
[localhost]:10023               (1400/postgrey)
[localhost]:10024               (24616/amavisd-new)
[localhost]:10025               (24566/master)
[localhost]:9001                (1018/php-fpm:)
[localhost]:10026               (24616/amavisd-new)
[localhost]:10027               (24566/master)
[anywhere]:587          (24566/master)
[localhost]:6379                (1078/redis-server)
[localhost]:11211               (1011/memcached)
[localhost]:9004                (1015/php-fpm:)
[anywhere]:110          (24671/dovecot)
[anywhere]:143          (24671/dovecot)
[anywhere]:465          (24566/master)
***.***.***.***:53              (24794/named)
[localhost]:53          (24794/named)
[anywhere]:21           (24782/pure-ftpd)
***.***.***.***:53              (804/systemd-resolve)
[anywhere]:5432         (1161/sshd)
[localhost]:953         (24794/named)
[anywhere]:25           (24566/master)
*:*:*:*::*:993          (24671/dovecot)
*:*:*:*::*:995          (24671/dovecot)
*:*:*:*::*:10024                (24616/amavisd-new)
*:*:*:*::*:10026                (24616/amavisd-new)
*:*:*:*::*:3306         (24198/mysqld)
*:*:*:*::*:587          (24566/master)
*:*:*:*::*:6379         (1078/redis-server)
[localhost]10           (24671/dovecot)
[localhost]43           (24671/dovecot)
*:*:*:*::*:8080         (840/apache2)
*:*:*:*::*:80           (840/apache2)
*:*:*:*::*:8081         (840/apache2)
*:*:*:*::*:465          (24566/master)
*:*:*:*::*:21           (24782/pure-ftpd)
*:*:*:*::*:5432         (1161/sshd)
*:*:*:*::*:25           (24566/master)
*:*:*:*::*:443          (840/apache2)

 

Version of Ubuntu running on that host is very old. Have you joined Extended Security Maintenance ESM? Otherwise I think 8 year old Ubuntu is no longer supported.
Is Roundcube installed from Ubuntu 18.04 repositories? If it is, it should work. If it is installed from somewhere else, it may well be the currently installed Roundcube no longer supports PHP 7.2.

 

You can remove Roundcube (you have not revealed what version it is now or from where it came from).
Then install previous Roundcube that worked OK, and pin it to that version so the automatic update(?) stops updating that package. Read apt and dpkg man pages to see how this is done. It is too early in the morning for me to remember the details. It might confuse ISPConfig when application version goes back, you may have to use Tools | Resync or manually verify configuration files ISPConfig writes for Roundcube.
It might be more straight forward to upgrade system to Ubuntu 20.04, and then to next LTS version. You should upgrade to Ubuntu 26.04 next autumn when ISPConfig starts supporting it.
Distro upgrade should work, unless you have modified the Ubuntu in some ways that break upgradability. Howtoforge. See if you can find info on how to upgrade with Internet Search Engines using

Code:

site:howtoforge.com upgrading ubuntu 18.04