1.3.9a installation using Paypal website payments pro

We are getting error emails from PayPal:

"Please check your server that handles PayPal Instant Payment Notification (IPN) messages. Messages sent to the following URL(s) are not being received:

https://www.brooklyntweed.com/store/ipn_main_handler.php"

This is the url that we have in our PayPal setup and the file does exist at that url.

I've run the IPNcheck script and it comes back OK:
IPNCHECK.PHP - Version 1.3.9

IPN TESTING - Response Received via CURL -- COMMUNICATIONS OKAY
IPN TESTING - Confirmation/Validation response with fsockopen(): FSOCKOPEN() RESPONSE RECEIVED - Communications OKAY
Default method likely to be used for communications is: CURL, with the fallback method being FSOCKOPEN if possible.
Script finished.

I'm stumped -- where else should we be looking?