Nginx - 56 day old reverse-proxy suddenly unable to connect upstream.

[J.R.]

> resolver 8.8.8.8 8.8.4.4 valid=3s ipv6=off;

I doubt this is related to your issue, but any reason you have 'valid'
set to only 3 seconds for your resolver conf? Seems like you could be
doing a lot of unnecessary repetitive lookups because that is set so
low.

> ssl_session_cache    shared:SSL:1m;
> ssl_session_timeout  86400;

This also seems dubious. SSL session timeout is set to 24 hours, but
your session cache is only 1MB.