http error 403 when redirecting to my website from another website

I am currently making a website where users can buy subscriptions by making payment online. To achieve this, a 3d secure payment gateway is being used. Some data is being sent via post from my website to the bank's website where the transaction is performed and the transaction result along with other variables are posted back to my website. For some reason after the transaction is complete and it tries to redirect to my site url, i get a http error 403 "Access to mysite.com was denied, You don't have authorization to view this page."

I tried the codes on a different site on the same server using plain php and the redirect works fine. But when it comes to my site I am getting errors, I assume it is something to do with codeigniter routing or access?

Any help will be greatly appreciated.

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