Hi Ilya
Did you set the mailfolders in the config file?
I'm also using Roundcube on Dovecot and never saw a problem like this.
I'm using the following config:
$rcmail_config['trash_mbox'] = 'Trash';
$rcmail_config['default_imap_folders'] = array('INBOX', 'Drafts', 'Sent', 'Junk', 'Trash');
cheers,
Claudio