thanks for the reply!
since the error message seems to involve the server, I tried changes in plugins/managesieve/config.inc.php in this section:
// managesieve server address, default is localhost.
// Replacement variables supported in host name:
// %h - user's IMAP hostname
// %n - http hostname ($_SERVER['SERVER_NAME'])
// %d - domain (http hostname without the first part)
// For example %n = mail.domain.tld, %d = domain.tld
$config['managesieve_host'] = 'localhost';
for $config['managesieve_host'] I tried all possibilities: 'localhost' '%h' '%n' '%d' '' without success, I still get «unable to connect to server.»
any other suggestion? does roundcube issue any error logs that I can look at?
Thanks and best regards, Meaulnes