Hi,
It's a bug. You can fix it by editing file 'application/models/mail_contents/MailContent.class.php', line 182 onwards, change from:
function clearEverything() {
$this->clearContentFile();
$this->clearMailData();
parent::delete();
}
to:
function clearEverything() {
$this->clearContentFile();
$this->clearMailData();
parent::clearEverything();
}