we received the following from Paypala:

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

https://lbps.net/j3site/index.php?option=com_j2store&view=checkout&task=confirmPayment&orderpayment_type=payment_paypal&paction=process&tmpl=component

following this link leads to the message:

Warning: Undefined array key "custom" in /home/lbps/j3site.lbps.net/plugins/j2store/payment_paypal/payment_paypal.php on line 986

The line in question reads:

   $postFields['PAYMENTREQUEST_0_CUSTOM'] = $vars->order_id.'|'.$vars->cart_session_id;

The result is that orders get marked 'failed' and we do not receive notifications of the order; we only learn of the order via PAYPAL's 'payment received' email, 

We are still using the 'legacy' version of j2store since the site is still running Joomla 5.4.6 - largely because one of our plugins uses the f0f library