Você está usando o Varnish como "proxy transparente" com " return (pipe) "?
Nesse caso, se você estiver usando return (pipe) terá que alterá-lo para retornar (passar) .
De link
pass
When you return pass the request and subsequent response will be passed to and from the backend server. It will not be cached. pass can be returned from vcl_recv
pipe
Pipe can be returned from vcl_recv as well. Pipe short circuits the client and the backend connections and Varnish will just sit there and shuffle bytes back and forth. Varnish will not look at the data being send back and forth - so your logs will be incomplete. Beware That with HTTP 1.1 client can send several requests on the same connection and so You should instruct Varnish to add a "Connection: close" header pipe before actually returning.