express.js sessions with reverse proxy
It should be trivial to look at the session cookie being forwarded and see why your browser doesn't re-send it when proxied -- the domain or path are likely wrong.
Re: your later comments -- Maybe you really did set "ProxyPassReverseCookieDomain / /" instead of using domains as the argument?
ProxyPreserveHost would probably also indirectly work for you, but fixing up the cookie is better.