Error PHP

PHP Fatal error:  Uncaught RuntimeException: Failed to start the session

Open the Varnish configuration file: 
/etc/varnish/default.vcl or /etc/varnish/varnish.params

You should see a line similar to the following in your configuration file:

# set the size of the cache in MB
set max_cache_size = 256M;

Increase the cache size:

set max_cache_size = 512M;

Save the configuration file and restart Varnish