The origional answer should not have been to delete/reinstall dovecot. Roundcube is a PHP script. If it couldn't access the storage engine, it was probably a permissions problem. If you reinstalled dovecot, but didn't configure it correctly, it has no idea what accounts are on the server. CWP uses Mysql to hold the account info, and if dovecot deleted that table on uninstall, you may have fubared your mail.