よく知らんけど変な話だなーと思ってぐぐってみたよ

http://www.roundcubeforum.net/5-release-support/17-pending-issues/5984-managesieve-mangled-folder-names.html

Set $rcmail_config['managesieve_mbox_encoding'] = 'UTF-8';

だとさ。これでどう？