VBScript runtime error '800a01a8' Object required

I started getting this error on our production Windows 2012 server running IIS 8.5:

Microsoft VBScript runtime error '800a01a8'

Object required: 'MERCHANTmydoc.documentElement'

The error seems to occur on this line:

Set MERCHANTmydoc=Server.CreateObject("Microsoft.xmlDOM")

No code was changed and the API has not changed (according to merchant support) when this started. I tested this same API string (inserting valid values) with Postman and got a valid reply, so I know the credentials work. I am beginning to suspect an issue with IIS, but don't know how to proceed.

MERCHANTstrXML = "<txn><ssl_merchant_ID>" & MERCHANT_ID & "</ssl_merchant_ID><ssl_user_id>" & MERCHANT_USER & "</ssl_user_id><ssl_pin>" & MERCHANT_PIN & "</ssl_pin><ssl_transaction_type>ccsale</ssl_transaction_type><ssl_card_number>" & merchant_card_number & "</ssl_card_number><ssl_exp_date>" & merchant_exp_date & "</ssl_exp_date><ssl_amount>" & merchant_trans_amount & "</ssl_amount><ssl_salestax>" & merchant_sales_tax & "</ssl_salestax>" & merch_indicator_var & "<ssl_cvv2cvc2>" & merchant_cvv2_code & "</ssl_cvv2cvc2><ssl_invoice_number>" & merchant_invoice_number & "</ssl_invoice_number><ssl_customer_code>" & merchant_customer_code & "</ssl_customer_code><ssl_first_name>" & merchant_first_name & "</ssl_first_name><ssl_last_name>" & merchant_last_name & "</ssl_last_name><ssl_avs_address>" & merchant_add1 & "</ssl_avs_address><ssl_avs_zip>" & merchant_zip & "</ssl_avs_zip><ssl_email>" & merchant_email & "</ssl_email><ssl_test_mode>" & MERCHANT_Test_Mode & "</ssl_test_mode></txn>"
        
if session("CHARGED") = "YES" then
    'Do nothing
else
    Set MERCHANTxmlhttp = Server.CreateObject("MSXML2.ServerXMLHTTP")

MERCHANTxmlhttp.Open "POST","https://api.convergepay.com/VirtualMerchant/processxml.do?xmldata=" & MERCHANTstrXML,false

MERCHANTxmlhttp.send 

MERCHANTresponsexml = MERCHANTxmlhttp.responseText  

Set MERCHANTmydoc=Server.CreateObject("Microsoft.xmlDOM")
MERCHANTmydoc.async= false
MERCHANTmydoc.loadxml(MERCHANTresponsexml)

I found a script to test xml requests and got this error: Sending XML data to http://localhost:8096/cart/receive3.asp msxml3.dll error '80072efd'

A connection with the server could not be established

/cart/post3.asp, line 12

These are the scripts I used

<%

    '// URL to which to send XML data
    URL="http://localhost:8096/cart/receive3.asp"

    Response.Write "Sending XML data to " & URL & "<br/>"

    information = "<Send><UserName>test</UserName><PassWord>user</PassWord><Data>100</Data></Send>"
    Set xmlhttp = server.Createobject("MSXML2.ServerXMLHTTP")
    xmlhttp.Open "POST", url, false
    xmlhttp.setRequestHeader "Content-Type", "text/xml" 
    xmlhttp.send information

    '// Report the response from the called page
    response.write "Response received:<hr/><span style='color:blue'>" & xmlhttp.ResponseText & "</span><hr/>"

%>

<%

    Dim objXmlRequest
    Set objXmlRequest = Server.CreateObject("MSXML2.DOMDOCUMENT.3.0")

    IF objXmlRequest.Load (Request) THEN

      'GET THE REQUEST FROM CLIENT
      strQuery = "//UserName"
      Set oNode = objXmlRequest.selectSingleNode(strQuery)
      strActionName = oNode.Text
      response.write "success! user name is " & strActionName  

    ELSE

        Response.Write "Failed to load XML file, reason: " & objXmlRequest.parseError.reason 

    END IF
%>