We too are now expeirencing this problem - started about 48 hours ago.
No problem with server all has been checked with provider. No changes at all have been made to the site to cause it. Now have recieved confirmation from Paypal that they are aware of the problem
Please check your server that handles PayPal Instant Payment Notifications (IPN). Instant Payment Notifications sent to the following URL(s) are failing:
http://www.#############.co.uk/index.php?option=com_hikashop&ctrl=checkout&task=notify¬if_payment=paypal&tmpl=component&lang=en&Itemid=118
http://###############/index.php?option=com_hikashop&ctrl=checkout&task=notify¬if_payment=paypal&tmpl=component&lang=en&Itemid=118
If you do not recognize this URL, you may be using a service provider that is using IPN on your behalf. Please contact your service provider with the above information. If this problem continues, IPNs may be disabled for your account.
Am going around in circles trying to get this sorted. Is anybody at Hikashop able to give a clear, easy to understand answer, on how to fix this?