We have Varnish as our load balancer and reverse proxy cache for normal HTTP traffic. For HTTPS traffic, we use Pound proxy to unwrap the SSL and forward to Varnish, which then forwards to the back-end servers. This is used for our "checkout" process to encrypt credit card info in transition. However, on the last stage of checkout, users are always getting an HTTP 500 (Internal Server) error. It doesn't seem to be due to our back-end app server, by all tests I've tried. Does anyone know anything about how that transition works-- the transition back from HTTPS to HTTP and the interaction between Pound and Varnish-- and why it might cause 500 errors?