The TCP state graph for the situation is:

https://i.imgur.com/USECPtc.png

You can see at 16:55 the FIN_WAIT1 ,CLOSE_WAIT and ESTABLISHED takes a steep climb, At this point Nginx hangs as the server has a script that checks stub status and this doesn't finish. The server itself and all other services like sshd works pretty fine ,except Nginx

At 17:06 you can see the connection states drop steeply too. this is the point where Nginx is stopped and Apache takes over as the webserver
The ESTABLISHED and CLOSE_WAIT drop fully and the FIN_WAIT1 reduces slowly and finally attain normalcy at 17:15

Apache continued working fine with the same amount of external traffic and same firewall setup