Erratic "PayPal Instant Payment Notification Warning"

Hi,

A client of mine is getting these PayPal mails rather often:

Please check your server that handles PayPal Instant Payment Notifications (IPN). IPNs sent to the following URL(s) are failing:

https://www.xxx.xxx/wp-admin/admin-ajax.php?action=mp_process_ipn_return_paypal_express

This happens on a multisite on https://m-etropolis.com running MarketPress in normal (not global) cart mode, using PayPal Express checkout on all sites.

This is the IPN history data he sent me of the most recent failed IPN on a sub-site with mapped domain :

Message ID
1WT29043LG807552E
Date/time created
14/02/2017 18:06 GMT+01:00
Original/Resent
Original
Latest delivery attempt date/time
15/02/2017 05:29 GMT+01:00
Notification URL
https://www.digitusimpudicus.org/wp-admin/admin-ajax.php?action=mp_process_ipn_return_paypal_express
HTTP response code
503
Delivery status
Retrying
No. of retries
13
Transaction ID
7AY23862V85750056
IPN type
Transaction made
IPN Message
mc_gross=100.00&invoice=0765108018c2&protection_eligibility=Eligible&address_status=confirmed&item_number1=Espace&payer_id=H3CW2NK8WW8UQ&tax=0.00&address_street=xxx&payment_date=09:06:03 Feb 14, 2017 PST&payment_status=Completed&charset=UTF-8&address_zip=xxx&mc_shipping=0.00&mc_handling=0.00&first_name=xxx&mc_fee=3.65&address_country_code=FR&address_name=xxx&notify_version=3.8&custom=284148486&payer_status=verified&business=xxx@xxx.com&address_country=France&num_cart_items=1&address_city=xxx&verify_sign=AgePMPfbDDc05TM5uouG2O9s9U74AmuSRKSnpoURH8ziF6ejPV818KSd&payer_email=xxx@xxx.fr&tax1=0.00&txn_id=7AY23862V85750056&payment_type=instant&last_name=xxx&address_state=&item_name1=Espace "ouvert" | Salon du Bien-Être&receiver_email=xxx@xxx.com&payment_fee=&quantity1=1&receiver_id=QS6SX55W4BH2U&txn_type=cart&mc_gross_1=100.00&mc_currency=EUR&residence_country=FR&transaction_subject=&payment_gross=&ipn_track_id=4fe47923df869

I've checked the server access (!) log (Apache, shared hosting at OVH.com) and found numerous entries reporting 503 responses, some 404 response and some 200 responses. The 200 (found) responses correspond with transactions where IPN history is shown in the MarketPress purchase details. The service unavailable and not-found responses (obviously) correspond with transactions where no IPN data is available in MarketPress purchase details.

The entry corresponding with above PayPal warning:

173.0.81.1 http://www.digitusimpudicus.org - [15/Feb/2017:05:29:50 +0100] "POST /wp-admin/admin-ajax.php?action=mp_process_ipn_return_paypal_express HTTP/1.1" 503 100 "-" "PayPal IPN ( https://www.paypal.com/ipn )"

Note that there are no related entries in the error log. The 503 response must be coming from MarketPress, not from the server.

Again, not all IPN communications fail. Some transactions are reported as payed instantly but others are failing for some reason. And subsequent IPN attempts for that same transaction seam to keep failing no matter what.

I should also mention:
1. This is happening on both the main site and on sub-sites with a mapped domain. Don't know about sub-sites without domainmapping...
2. All domaines are included in one SSL license used accross the network. The license is issued by Let's Encrypt.

How to get stable IPN?