Why does nginx give the page every other time?

Question

Nginx

Debian 9. Nginx as a proxy to Apache. Gives clearly through time. For the second time 404.

server {
listen 80;
server_name domain.ru;
location / {
proxy_pass http://domain.ru:8080;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $scheme;
}
}

Without nginx everything works perfectly.

Answer 1

And "serv_ip" is an IP and not a domain

upstream upserv {
              least_conn;
              server serv_ip:8080 weight=10 max_fails=3 fail_timeout=30s;
        }
        
  server{
           listen 80;
           server_name domain.ru;
          
          location / {	  
           proxy_pass http://upserv;
           proxy_http_version 1.1;
           proxy_set_header Upgrade $http_upgrade;
           proxy_set_header Connection 'upgrade';
           proxy_set_header Host $host;
           proxy_cache_bypass $http_upgrade;
              
        }
}

Answer 2

look at the logs.
if there is nothing, then turn on the debug and look again.
besides, you have a very strange config. why hang up apache on external ip? are you going to connect to it bypassing nginx? Upstream is not needed if you have one.