Paypal Invalid Response

Hello, My site has received this message:
"
Hello,
A paypal notification was refused because the response from the paypal server was
invalid
Check the documentation concerning this issue at
www.hikashop.com/index.php?option=com_do...rror#invalidresponse
This notification was for the order J8 on the website ozdetox.com.au/
You can access the order details directly by clicking on the link below after
logging in your back end:
ozdetox.com.au/administrator/index.php?o...task=edit&order_id=8
"

The payment went through though.. does it have something to do with paypal being down/upgraded a few days ago?

Please advise how to fix this

Cheers
Matt