The prestashope  software need big header size to pass http. So please do the folowing to fix this issue,

1)  Edit  the vhost.conf   ( under /etc/cpnginx or /etc/danginx )  have the following too

client_max_body_size    100m;
client_body_buffer_size 512k;
proxy_send_timeout   90;
proxy_read_timeout   90;
proxy_buffer_size    32k;
proxy_buffers     16 32k;
proxy_busy_buffers_size 64k;
proxy_temp_file_write_size 64k;
proxy_connect_timeout 60s;

2) You may need to add the following two lines to the Main Nginx configuration file

client_header_buffer_size 16k;
large_client_header_buffers 16 16k;

So after that the nginx configuration may look like the following,

# cPanel Nginx Master configuration
user nobody;
error_log logs/error.log;
#Number of worker you need
worker_processes 1;
# How many connections a worker can handle maximum.
events {
worker_connections 50000;
}
http {
include mime.types;
default_type application/octet-stream;
sendfile on;
server_names_hash_max_size 5000;
server_names_hash_bucket_size 128;
#Prestashope need the following two lines

client_header_buffer_size 16k;
large_client_header_buffers 16 16k;

tcp_nopush on;
tcp_nodelay on;
keepalive_timeout 0;
gzip on;
gzip_min_length 1100;
gzip_buffers 4 32k;
gzip_types text/plain application/x-javascript text/xml text/css;
ignore_invalid_headers on;
client_header_timeout 3m;
client_body_timeout 3m;
send_timeout 3m;
include “/usr/local/nginx/conf/vhost.conf”;
include “/etc/cpnginx/cpanelproxy.conf”;
}

After modifying this , you may need to restart the Nginx server.