No worry about out-of-topic discussion, i can split the post to a new forum topic.
If Roundcube webmail cannot connect to managesieve server, please check its log file first, it should be the same as Postfix log file. For example, /var/log/maillog, or /var/log/mail.log, or /var/log/mail.
You can execute below command to check whether managesieve service is running or not:
In old versions, managesieve listens on port 2000, but it's 4190 on some new OS releases.
If managesieve is running, please check Roundcube managesieve plugin config file, make sure it connects to correct port. For example: the config file is /var/www/roundcubemail/plugins/managesieve/config.inc.php on RHEL/CentOS/Scientific Linux.
$rcmail_config['managesieve_port'] = 4190;