Customer gets HTTP 500 error after order is placed -- after upgrading

HI I have a good store based on zencart at https://www.arunachala.org/bookstore It is based on version 1.3.9f I tired to upgrade it to 1.3.9h The pay pal was removed and installed. Since then I am having problem after an order is completed. The order goes thru but the customer gets a HTTP 500 error. Currently I have put the website on maintenance. Even after backing up to an older version the problem persists. I suspect the return arn in Pay Pal could be the problem. It is currently set to https://www.arunachala.org/bookstore/index.php?checout_process

Kindly help. Donuts and coffee for the wizard!

Starting point for diagnosing Internal Server errors.

I solved the problem. There was a parse error in the header_php.php of checkout_success. When I fixed it the problem disappeared. The debug error log in cache was helpful in finding the location of phps parse error. The mystery remains why was it working fine for over two years. The answer may lie in the fact that only yesterday I corrected the redirect from Pay Pal which was going to the wrong place. That raises the interesting question about how the check out page was appearing correctly when the redirect was going to wrong website!! I guess life will not be exciting without mysteries. All that ends well is well:clap: