First, make sure Request.Form ["openid_identifier"] returns a valid URL (openid path to yahoo or google). If this is not the case, you have a problem with your openid selector. If so, the request is executed, but the openid provider does not respond on time, because of which you get "OpenID Endpoint not found". Try sending the request again and see if it will work for the second subsequent time.
UPDATE
If you are working under a proxy server, you need to indicate that
Example:
<defaultProxy useDefaultCredentials="true"> <proxy autoDetect="True" usesystemdefault="True"/> </defaultProxy>
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