I am supporting a Classic ASP application that connects to a payment gateway via HTTPS. Up until recently there have been no issues. A few days ago this broke without the code, IIS config or anything local changing. Its broken on at least 3 separate servers. The last run of Windows Updates was in late November, but bringing the servers' updates up date has not resolved the problem. A code snippet is below.

Dim oHttp
Dim strResult
Set oHttp = CreateObject("MSXML2.ServerXMLHTTP")
oHttp.setOption 2, 13056
oHttp.open "POST", SOAP_ENDPOINT, false
oHttp.setRequestHeader "Content-Type", "application/soap+xml; charset=utf-8"
oHttp.setRequestHeader "SOAPAction", SOAP_NS + "/" & SOAP_FUNCTION
oHttp.send SOAP_REQUEST

Below is a dump of the error object :-

Number: -2147012852
Description: A certificate is required to complete client authentication 
Message: A certificate is required to complete client authentication

I initially posted the question on Stackoverflow (http://stackoverflow.com/questions/9212985/cant-use-https-with-serverxmlhttp-object) thinking it was a code issue, but further investigation seems to point to a server issue.