'StatusDetail= Cannot match the MD5 Hash. Order might be tam

Hi all.

We're using Opencart 2.0.1.1

When making a purchase via Sagepay Server - and picking the 'PayPal' option, we are hitting this error:
This check starts at line 345 in sagepay_server.php:

if ($str_my_signature != $str_vps_signature) {
$this->model_payment_sagepay_server->deleteOrder($order_id);

echo "Status=INVALID" . $end_ln;
echo "StatusDetail= Cannot match the MD5 Hash. Order might be tampered with." . $end_ln;
echo "RedirectURL=" . $error_page . $end_ln;

$this->model_payment_sagepay_server->logger('StatusDetail= Cannot match the MD5 Hash. Order might be tampered with.');

return;
}

Note, the payment transaction works successfully. We can take payments via this route - but the user is not getting redirected back properly to the site.

Instead, they get this error message:
Has anybody got any experience with fixing this? There seems to be a mismatch between MD5 hashes, but I'm not sure the correct route to fix this.

Thanks.