I've updated ZCart from 1.3.9 to 1.5.1 and found the next:

in v 1.3.9 I didn't select radio button currency payment in PayPal section so I could receive payments in different currencies on my PayPal account(in my case NZD)

in v1.5.1 radio button MUST BE selected(in my case USD, because there is no option for NZD), otherwise payment with PayPal processed Ok, IPN history record has been created Ok, but Order record is not created at all and has ID=0 for IPN history record, no emails created either