Roundcube filter error on a Plesk server: File is missing, please contact server administrator

Symptoms

• The following error can be seen when accessing Roundcube Webmail > Settings > Filter (Out of Office, or Forwarding) you get the following error

  FILE NOT FOUND
  The requested resource was not found!
  Please contact your server-administrator.
  Failed request: webmail.example.com/roundcube/?_task=settings&_action=plugin.managesieve

• You see the following error when in /var/log/plesk-roundcube/errors:

  [01-Jul-2024 19:41:39 +0000]: <je5n10vf> PHP Error: Error loading template for managesieve.managesieve in /usr/share/psa-roundcube/program/include/rcmail_output_html.php on line 804 (GET /roundcube/?_task=settings&_action=plugin.managesieve

Cause

This is due to a skin being used on webmail:

# grep skin /usr/share/psa-roundcube/config/config.inc.php | grep config | head -n2
$config['skin'] = 'custom_skin';

Resolution

1. Log in using SSH.
2. Use a text editor to comment out the custom skin line on configuration file /usr/share/psa-roundcube/config/config.inc.php. The line should start with $config['skin'].