Hmmm... when I verified my files against one_page_checkout-2.3.7.zip, my OnePageCheckout.php was a match. Here is line 2323 from my OnePageCheckout.php:
which looks like the error message I received.Code:trigger_error("Unknown/invalid billto address #{$_SESSION['billto']} for customer#{$_SESSION['customer_id']}.", E_USER_ERROR);
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