WordPress returning 500 Internal Server Error from Banks remote server in Payment Gateway

I am trying to implement HDFC Bank's Payment Gateway on WordPress. Here is the process instructed by the bank:

Step 1: Send request to server via curl if connection is successful and authenticated PG sends PaymentID, which I have to update in database, after that redirect user to PG's site to perform transaction.

Step 2: After a transaction is successful or unsuccessful PG sends response back to my site on the url which is registered as response url of my site with the server e.g. http://ift.tt/1CYaYtC And I have to process that response on that page/url on my site. But they say that my WP site throws 500 Internal Server Error when bank's server try to send response to the given registered response url.

I tried every possible solution after searching net to stop this behaviour:

Placed bank's IP addresses in .htaccess by alllowing them,

increased memory limit via php.ini,

installed WP htaccess Control Plugin and tried every possible option given in that,

I tried on Fresh WP installation without installing any plugin just applied PG logic inside a custom page template,

but the situation remains the same.

First I tried it on WP version 3.9.1 after that I updated WP to the latest v 4.1, but bank's server still getting 500 error from my WP site.

Most importantly: when I try to fetch server's response through PHP( not fetching via WordPress), response is received properly.

It means there is no problem with my Web Host(hostgator). And I think there is no need to dig deeper inside the code because WP is not processing the response received at all, it just throwing 500 Internal Server error to the PG Server.

Any Suggestion to tell WP not to through 500 error on that page when accessed from another server??

One more thing: When response url is accessed via browser directly, there is no error at all. WP processes url as normally it should.