While I can't explain the root-cause of those logs, it looks like changing line 216 of /includes/classes/observers/class.checkout_one_observer.php to read as follows should correct the situation.
Code:
        if (!(isset($_SESSION['opc_error'], $_SESSION['opc']) && $_SESSION['opc_error'] === OnePageCheckout::OPC_ERROR_NO_JS) && $_SESSION['opc']->guestCheckoutEnabled() === true) {