It looks like there's a conflict with the eCart redirect 'header' line and the ProductDb.php file. Both lines of code are cited in the error message.
<?php
// WA eCart Redirect
if ($eCart1->redirStr != "") {
if (function_exists("rel2abs")) $eCart1->redirStr = rel2abs($eCart1->redirStr,dirname(__FILE__));
header("Location: ".$eCart1->redirStr);
die();
}
?>
ProductsDb.php file attached, with others.