Roundcube seems to trying port 2000 to access dovecot managesieve and fails, as that is not configured.
/etc/roundcubemail/managesieve.inc.php says it is optional to specify the port, but, there is no example of how to specify the port.
Examples on the internet suggest this should work, but apparently does not.
$config['managesieve_port'] = 4190;
$config['managesieve_host'] = 'localhost';
How is it to be done?
Edit/Modify:
Apparently, like this:
$config['managesieve_port'] = 4190;
$config['managesieve_host'] = 'localhost:4190';
No idea if the first line is really necessary, I think not, but, not inclined to test it at the moment.