I've just enabled Varnish on a site using the Turpentine extension. Everything's fine except I have a login form in the header.
If I use this form on a unsecure page, then it redirects me to the normal login page. If I use this form on a secure page, it works as expected and the customer is logged in.
Everything was working fine before I enabled Varnish.
The login form has the action "https://..../customer/account/loginPost/"
Any ideas what I can change to get this to work?