The error log you posted I dont think is for tyhe problem you are encountering with eCart, it must be from another script, it is reporting the referer as skyyomer.php.
The Internal server error is a very generic error, We will need more infomation to be able to get an idea of the cause. You can try adding the following code to line of the cart.php page to turn error reporting on:
<?php
error_reporting(E_ALL);
ini_set('display_errors','on');
?>
if your host allows, this will raise the error reporting level and hopefully return a more helpful error,.