Nginx error: worker process 12345 exited on signal 6 (core dumped)

I habe a problem with nginx. Sometimes, when a user visit my site, the connection are lost, and i don't know why!? The site runs with https:// provided by lets encrypt.
I gues the problem comes from the ssl connection, but can not find solution.
The error repeats itself exactly every minute

Here the errors:

2016/07/21 12:07:07 [alert] 13540#13540: *847 open socket #14 left in connection 16
2016/07/21 12:07:07 [alert] 13540#13540: *942 open socket #32 left in connection 22
2016/07/21 12:07:07 [alert] 13540#13540: aborting
2016/07/21 12:07:07 [alert] 13537#13537: worker process 13539 exited on signal 6 (core dumped)
2016/07/21 12:07:07 [alert] 13537#13537: worker process 13540 exited on signal 6 (core dumped)

My nginx version is 1.10.1.

My nginx.conf:
===========
user www-data;
worker_processes 4;
pid /run/nginx.pid;
worker_rlimit_nofile 65535;
debug_points abort;

events {
worker_connections 4096;
multi_accept on;
use epoll;
}
http {
charset UTF-8;
sendfile on;
tcp_nopush on;
tcp_nodelay on;
keepalive_timeout 10; #65
types_hash_max_size 2048;
server_tokens off;

include /etc/nginx/mime.types;
default_type application/octet-stream;

access_log off;
error_log /var/log/nginx/error.log crit;
error_log /var/log/nginx/error.log error;

gzip on;
gzip_disable "msie6";
gzip_static on;
gzip_http_version 1.1;
gzip_vary on;
gzip_comp_level 6;
gzip_proxied any;
gzip_types text/plain text/css application/json application/x-javascript text/xml application/xml application/xml+rss text/javascript application/javascript text/x-js;
gzip_buffers 16 8k;

open_file_cache max=5000 inactive=20s;
open_file_cache_valid 30s;
open_file_cache_min_uses 2;
open_file_cache_errors on;

include /etc/nginx/conf.d/*.conf;
include /etc/nginx/sites-enabled/*;
}

My vhost.conf
===========
server {
charset UTF-8;
listen 443 ssl;
server_name website.de;

ssl_certificate /etc/letsencrypt/live/website.de/fullchain.pem;
ssl_certificate_key /etc/letsencrypt/live/website.de/privkey.pem;

ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
ssl_prefer_server_ciphers on;
ssl_dhparam /etc/ssl/certs/dhparam.pem;
ssl_ciphers 'A value...';
ssl_session_timeout 1d;
ssl_session_cache shared:SSL:50m;
#ssl_session_cache none;
ssl_stapling on;
ssl_stapling_verify on;
add_header Strict-Transport-Security max-age=15768000;

gzip on;
gzip_http_version 1.1;
gzip_vary on;
gzip_comp_level 6;
gzip_proxied any;
gzip_types text/plain text/css application/json application/x-javascript text/xml application/xml application/xml+rss text/javascript application/javascript text/x-js;
gzip_buffers 16 8k;
gzip_disable "MSIE [1-6]\.(?!.*SV1)";
}

Any ideas, where is the problem?

Thanks
Frank