Thank you, John.
You're so right, I've read too much posts and articles about sieve (I'm also new to sieve:confused:), and tried too many inappropriate settings. This is one of those...
I found both
$rcmail_config['sieverules_host'] = 'localhost';
and
$rcmail_config['managesieve_host'] = 'localhost';
had been set, so deleted the second one.
Any other things you think of?