Please see screenshot attached here and I face this problem in Magento 2, so please suggest me if anyone to how to fix?
I had the same issue, I was using the magento2devbox. The problem came when I modified a model (apparently was buggy). As for the solution I modified to this in the /etc/default/varnish:
DAEMON_OPTS="-a :6081 \ -T localhost:6082 \ -f /etc/varnish/default.vcl \ -p thread_pools=4 \ -p thread_pool_max=1500 \ -p connect_timeout=300 \ -p http_resp_hdr_len=65536 \ -p http_resp_size=98304 \ -S /etc/varnish/secret \ -s malloc,2G"
And restarted the web server and varnish server. If the model is kept buggy you will get this problem again (if this is the trigger) and then you have to restart the web server again and again until you fix it.