Error Processing Checkout.. Please Try Again

Hi guys, my website is showing an error on my checkout page.
I’m not able to troubleshoot this issue even though I tried many different things.
Do you have any ideas what might be causing it?

The error is showing only when I choose a specific payment gateway.
When I click order now is supposed to redirect me to the payment gateway’s url, instead this error is shown.

Thanks in advance.

The page I need help with: [log in to see the link]