PayFast payment gateway set up failed

Hi All

I need assistance with this urgently.
Here is the breakdown:

I have a self-hosted solution, I used Softaculous to install Invoice Ninja
I have configured everything but the last step is to set up the online payment gateway

As I am in South Africa, I used PayFast as I have an existing account with them.

When I insert my Merchant ID & Key, and hit save it takes me to a 403 error.

Please could someone assist me to resolve this.

Please check for details about the error in the web server error logs.