InvalidAuthenticityToken after updating JSP

I just merged the latest changes of JSP. And I ran into several problems. The one persisting is, that I get an error when trying to sign in:

ActionController::InvalidAuthenticityToken (Can't verify CSRF token authenticity.):

web    |   

web    | lib/jumpstart/lib/jumpstart/account_middleware.rb:30:in `call'

I tried different browsers and deleted existing cookies, but no luck. What am I missing?