Classic ASP, ServerXMLHTTP authenticates on old server, not on new one

[muffin]

I'm working with a classic ASP website on a Windows 2003 server. The site requires authentication (configured to use Basic authentication). One page uses ServerXMLHTTP to request data from another page on the same website (same protocol [HTTPS], same host name), using the Open method with the arguments Method, URL, Async, UserName, Password. It's been working correctly for years. When the page that makes the server-side call is requested, the log shows one request, with no user name, for the back-end page that returns a 401 status, and then another request, that includes the user name, that returns a 200. These are followed by the request for the front-end page, with a 200 status.I've copied the site to a Windows 2008 R2 64-bit server, with apparently the same authentication configuration in IIS. But now no data is received from the back-end page. The log shows TWO consecutive requests for the back-end page with no user name, and BOTH return a 401, and then the request for the front-end page, with a 500 status.What could the problem be? I've seen discussions of proxycfg, but we aren't using a proxy. I've seen discussions of also setting credentials using Base64, but (a) we don't do that on the old server and (b) I tried it anyway, and it didn't make any difference.