Yes, I presume it's a php version problem.
The error is PHP Fatal error: Call to a member function getApiResult() on null in /...../ext/modules/payment/paypal/pro_hosted_ipn.php on line 28
Line 28 is
$result = $paypal_pro_hs->_app->getApiResult('APP', 'GetTransactionDetails', array('TRANSACTIONID' => $HTTP_POST_VARS['txn_id']), (OSCOM_APP_PAYPAL_HS_STATUS == '1') ? 'live' : 'sandbox', true);
Old server Php 5.2.11 - MySQL 5.0.88
New server is Php 5.6.29 - MariaDB 10.0.28
PayPal is trying to update the orders with new information about refound etc.