It seems that in the configuration file of Nginx (nginx.conf), you have "worker_connections 4096;", but on your server machine, there is hard limit of 1024 on open file resource.
If you can SSH to your server, run "ulimit -n" on the terminal to see the limit on open files. You can run "ulimit -a" to check other limitations. If the limit on open files is 1024, make the following change in nginx.conf file: "worker_connections 1024;"
Although Nginx documentation says that worker_rlimit_nofile can be used to increase the limit, but it never worked on my VPS.