Getting blank response from Payment Gateway post after re installing IIS in MVC

See more:

ASP.NET

IIS

MVC

Hello All,

Before Re installing IIS in server. Everything works fine. Once IIS re installed in server, site gone through some issues.

1) Site stop bringing CSS/Images without login. (Solved)
We have grant some additional permissions in website folder and its working fine now.

2) Site stop getting response from Payment Gateway (Unsolved)
Till now it works fine, we are getting proper response from payment gateway using which we can apply logic in our site.
But now, payment deduct successfully. but we get blank response in notify url, so we can not able to apply site logic in notify url.

Please instruct possible scenario.
Website developed in MVC.

Till now we are using Request.Querystring["TransactionID"] at notify url, and based on it we can successfully apply logic. But some how due to some permission revoke or any issue, we get null/blank in Request.Querystring["TransactionID"].

P.S. It's already confirmed from payment gateway that there is no issue from their site. also they stated that even we found blank response from your side.

According to them they are posting below data:
Content-Type='application/x-www-form-urlencoded';Host='website.com',Connection='close';

Content:
Amount=$$&MerchantReferance=XXX&&TransactionID=ddf-dfgfgf-fgfdg

Please help me out, it's big stuck for us. As this is live site.