Additional info, system logs show several:
kernel vm_thread_new: kstack allocation failed
And several
kernel sonewconn: pcb 0xc7274790: Listen queue overflow: 193 already in queue awaiting acceptance (1 occurrences)
nginx 2020/06/12 12:39:47 [error] 937#100185: *5059 connect() to unix:/var/run/php-fpm.socket failed (61: Connection refused) while connecting to upstream, client: xx.xx.xx.xx, server: , request: "GET / HTTP/1.1", upstream: "fastcgi://unix:/var/run/php-fpm.socket:", host: "xx.xx.xx.xx:xxxx"