Checkout blank page during checkout - authorize.net

I am having a similar issue with checkout suddenly not working. Zencart Version 1.3.9 using Authorize.net for checkout.
Once you go through the order process and fill in payment info and click on continue - a BLANK page is coming up (yet it process the order). Clients are then hitting refresh 2 or 3 times and the SAME ORDER is being placed 2,3,4 times. Payment is going through that amount of times too.

Have looked in the cache folder and there are lots of files for Authorize.net but have no idea what any of them mean.

The site is https://www.passthetexes.com
The page it is meant to go to when the order is complete is
https://www.passthetexes.com/download.php

Customers are freaking out. Though I did not custom code this site - just took it on for a client.

Attached is one of the error logs.