WinHTTPRequest - A certificate is required to complete client authentication

I have an API that has been working fine for several months but this past week we have had two instances where we have been unable to run our API calls and I think the error being returned is : A certificate is required to complete client authentication. The application is built on Classic ASP using WinHTTP (WinHTTP.WinHTTPRequest.5.1).

The first time this error happened, the service started to work again a few hours later. Today, it has not worked at all and is down completely.

My recipient account number is : CGB506463