When I tried to log in with my Yahoo! account, I got the familiar warning, so I've read through various documents. I've created an XRDS file: https://jck.golgotha.org.uk/WebRPApplication1/xrds.xrd and I've verified that this has the right content type ("application/xrds+xml"). I'm advertising it through a custom HTTP header ("X-XRDS-Location") on Default.aspx.
When I try to login, I get three relevant entries showing up in my firewall log (from a Yahoo! server): 1. Accessing the main folder. 2. Retrieving the xrds.xrd file. 3. "A connection was gracefully closed in an orderly shutdown process with a three-way FIN-initiated handshake."
However, in my web browser I then get an error message: "An error occurred and you will not be able to use your Yahoo! OpenID for this site." This is worse than the old warning, because there's no way for me to continue. However, I don't know what's causing this error. Is there any way to get more detailed diagnostics?