I found the fix to make the Product List Feed work, here are the steps needed to fix the issue.
1. Go to your-blog.com/wp-admin and login
2. Go to Plugins > Editor
3. In the list box labeled Select plugin to edit select WP Shopping Cart and press the select button
4. The WP Shopping cart files will appear, now click the second file in the list wp-e-commerce/wp-shopping-cart.old.php
5. On your web browser select the Edit Menu > Find
6. Enter header(‘Content-Disposition: inline; filename=”E-Commerce_Product_List.rss”‘); and press Enter (Return)
7. Right after this line enter in the follow code.
8. /*RSS Fix from https://www.w3it.org/blog/wordpress-feed-error-output-solution-how-to/*/
$out = ob_get_contents();
$out = str_replace(array(“\n”, “\r”, “\t”, ” “), “”, $input);
ob_end_clean();
9. Done! Now you’ll need to clear your web browsers cache so it will work. In Firefox go to Tools > Clear Recent History and only select Everything and Cache. In Internet Explorer go to Tools > Internet Option and select Temporary Internet File and press the Delete button.
Let me know if you have any issues!
Thanks,
Virgil