$ su nginx --shell /bin/bash --command "ulimit -n"
4096

it not fix the problem

nginx 10062 0.0 44.5 14763332 14639972 ? S 14:56 0:00 nginx: worker process
nginx 10063 0.0 44.5 14763332 14639964 ? S 14:56 0:00 nginx: worker process
nginx 10064 0.1 44.5 14763332 14639984 ? S 14:56 0:00 nginx: worker process
nginx 10065 0.1 44.5 14763332 14640880 ? S 14:56 0:00 nginx: worker process

have you more advice?