ZenCart 1.3.9g

I have hunted the threads and found similar issues, but no answers that have solved the problem for me.

When checking out, whether the PayPal Express Checkout button or the ZenCart Go To Checkout button is used, the checkout process goes all the way to PayPal where I log in to confirm the order. When I hit confirm in PayPal I am taken back to the PayPal login page where I have to login again and confirm again and the order goes through.

I turned off the PP EC Button in the admin panel and get the same result. cURL is set to FALSE as suggested in another thread. Is there anything else to check?

The site has been running fine all year until this month when customers brought this bug to our attention. So, nothing was modified prior to this occurring.

Thanks in advance for any suggestions or thread links that may help.