POST using MSXML2.ServerXMLHTTP60 used to work now ERROR

Question

I am using MSXML object to POST data from an Excel worksheet to a REST API. My VBA macro used to work, now MSXML2.ServerXMLHTTP60 returns an MS error "No credentials were available in the client certificate." The POST request, URL, credential and body all work fine when testing with Postman but not with MSXML2.ServerXMLHTTP60 so the API owner tells me it is a problem with VBA. My VBA macro works fine with a commercial test site that does not require authentication. I found some help that was mostly gibberish to me that seemed to indicate an MS installation registry key had become corrupt???? MS Office Support was of ZERO Help.

1. IS this a problem with MSXML2.ServerXMLHTTP60 or the API?
2. What causes this error when using secret credentials with MSXML2.ServerXMLHTTP60?
3. How to restore the previous functionality?
4. Please explain step by step like I'm a 7 years old.

cheers,
Rick