What's even more strange is that if the cart contents "total" is $0, the Express Checkout button isn't supposed to be shown at all.
From which page were you clicking on the Express Checkout button? Was it the Shopping Cart page or the Login page ?
Probably can fix it by editing /includes/modules/payment/paypal/tpl_ec_button.php
line 12 should read:
Code:
if ($ec_enabled && ($_SESSION['cart']->count_contents() > 0 && $_SESSION['cart']->total > 0)) {
and similarly in /includes/modules/pages/login/header_php.php
line 121:
Code:
$ec_button_enabled = ($paypalec_enabled && ($_SESSION['cart']->count_contents() > 0 && $_SESSION['cart']->total > 0));
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