I've tested a PayPal order (using standard One Page Checkout and core Paypal Payments Standard) when IPN is enabled and the statuses update fine so IPN seems to function.
However, PayPal sends through this email periodically:
Please check your server that handles PayPal Instant Payment Notifications (IPN). Instant Payment Notifications sent to the following URL(s) are failing:
http:// xxxx /paypal/ipn/
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.
Thank you for your prompt attention to this issue.
Yours sincerely, PayPal
and then after about a week automatically disables IPN which stops PayPal orders from updating properly.
Checking system.log shows a bunch of notices
2014-02-12T20:29:54+00:00 ERR (3): Notice: Undefined index: invoice in /var/www/windswept/app/code/core/Mage/Paypal/Model/Ipn.php on line 171
There are about 50 of these errors ( one every 15/20 min) while we had a small amount of Paypal orders recently.
Does anybody have any insight into this issue?