Installation problem

I’m getting this error message on install:

Error: it seems you tried to skip a step of the Installation process, or your cookies are disabled, or the Piwik configuration file was already created.
Make sure your cookies are enabled and go back to the first page of the installation .

I’ve seen this has happened to others. Session cooked is on:

(see phpinfo below)

I am running nginx with a varnish cache and suspect this might be the problem. any suggestions?

ession.auto_start Off Off
session.bug_compat_42 Off Off
session.bug_compat_warn Off Off
session.cache_expire 180 180
session.cache_limiter nocache nocache
session.cookie_domain no value no value
session.cookie_httponly Off Off
session.cookie_lifetime 0 0
session.cookie_path / /
session.cookie_secure Off Off
session.entropy_file no value no value
session.entropy_length 0 0
session.gc_divisor 1000 1000
session.gc_maxlifetime 1440 1440
session.gc_probability 0 0
session.hash_bits_per_character 5 5
session.hash_function 0 0
session.name PHPSESSID PHPSESSID
session.referer_check no value no value
session.save_handler files files
session.save_path /var/lib/php5 /var/lib/php5
session.serialize_handler php php
session.use_cookies On On
session.use_only_cookies On On
session.use_trans_sid 0 0