Why is my upstream localhost? Getting 500 but no app error in Nginx. " no live upstreams while connecting to upstream"
Solution 1:
Try to replace https with http here:
proxy_pass https://localhost:3000;
and reload nginx. You said that curl localhost:3000 worked, and omitting a protocol for curl assumes http://.
Most probably, your backend application does not support HTTPS (or not at this port, at least).