HTTPS keep-alive functionality is partially broken. Basically what happens is that when we are using the cached HttpsClient, we are still trying to do a SSL handshake.

Because of this and the behaviour of IIS 5.0, we can't do two connections to
the same host back to back using HTTPS keep-alive with IIS5.0 as server. The application would eventually receive an IOException with a response code 400, and need to reconnect and resend the http request again.

The problem wasn't discovered earlier because with most servers (IIS5.0 is the only server where we see http request failures) the client and server can resume an existing session and continue with the http request and response, although at a performance cost.