Here's the issue I have now.
After listening to suggestions from WHMCS on how the problem was "a server issue based on CentOs 6.5 + OpenSSL", I became desperate.
With no-one able to offer a true solution, I asked WHMCS support if I switched to a new server running CentOs 5 if this would solve the issue. They assured me this would indeed fix the problem.
So, last night, I paid for a new domain, migration and testing to ensure that my domain was all working. I then modified the nameservers to point to the new dedicated ip on the new server.
I update my WHMCS license on the new server ip, so far all is fine. I goto my process CC link and voila! NO DIFFERENCE.
I get the exact same error as before! If the issue is indeed because of CentOS 6.5, why does it not work still?