ActiveMerchant::ConnectionError: The remote server reset the connection - Transaction failure in sandbox

Hello,

Hope someone is able to help with this issue - I've been told that all questions regarding all First Data products could be discussed here. We've had a working integration with FirstData Virtual Terminal/Linkpoint for a few years. Within the last few weeks, when we're testing transactions on our sandbox, the following is returned after completing a transaction:

"ActiveMerchant::ConnectionError: The remote server reset the connection"

We've tried re-downloading the pem certificate from https://www.staging.linkpointcentral.com/ and re-uploading in our app, but still see the same error returned. Unsure what possible issues could have caused this.

Looking forward to some suggestions/ideas on how to troubleshoot further.
Thanks!