Click on PayPal Checkout goes directly to checkout_failure.php

I have above problem on 2 sites while trying to checkout with PayPal - one is using eCart 5 and one is using eCart 6.

For the eCart 5 site = Clicking on the "Checkout with PayPal button leads directly to the pp_checkout_failure.php page. I have included the paypal credentials in the checkout.php page as well as in the pp_confirm.php page

For the eCart 6 site = After choosing PayPal as a payment option, entering the shipping information and clicking on the continue button = leads to a redirect to the checkout_failure.php page. I have included the paypal credentials in the Gateway Credentials server behavior in the confirm.php page.

I also called PayPal to confirm our API Signature Credentials such as Username, Password and Signature. They seem to be correct.

What am I missing?

Thanks for any ideas.

Joerg