Troubleshoot Quikstore payment gateway failures

A client is running QuikStore v2.12 on his web site. A couple of weeks ago the credit card authorization and capture has stopped working. I am not sure if there was a change in the API or if something else is wrong. I need someone to troubleshoot this. The script is written in Perl, and the payment gateway is Linkpoint (Card Services International). The script has a plugin for this payment gateway and it was working fine for the last 3 years. I hope this is a simple issue, I will be willing to pay more (bonus) if the work gets more complicated.