Setting up nginx reverse proxy - 500 Internal Server Error

I'm trying to set up a reverse proxy server using nginx 1.20.1 on Fedora Server 35, but everything I've tried so far results in an error:

[root@proxy nginx]# curl -I -H "Host: www.example.com" http://10.0.19.1/
HTTP/1.1 500 Internal Server Error
Server: nginx/1.20.1
Date: Wed, 10 Nov 2021 19:54:39 GMT
Content-Type: text/html; charset=UTF-8
Connection: keep-alive
X-Powered-By: PHP/8.0.12

10.0.19.1 is the address of the nginx server, and 10.0.20.1 is an Apache server which I'm trying to proxy. If I run the same curl command as above, but using 10.0.20.1 for the URL, it works fine, so I think it's safe to rule out connectivity issues between the two servers, and any problem with the Apache host. The nginx error log is empty.

nginx -t Finds no problems with the config files (reproduced below). I also set SELinux to permissive for now to eliminate that as a cause.

I did create a simple (non-proxy) server configuration to test, and that worked OK, so being new to nginx, I get the feeling this is a probably a very trivial problem with my proxy.conf file.

nginx.conf:

user nginx;
worker_processes auto;
error_log /var/log/nginx/error.log warn;
pid /run/nginx.pid;

include /usr/share/nginx/modules/*.conf;

events {
    worker_connections 1024;
}

http {
    log_format  main  '$remote_addr - $remote_user [$time_local] "$request" '
                      '$status $body_bytes_sent "$http_referer" '
                      '"$http_user_agent" "$http_x_forwarded_for"';

    access_log  /var/log/nginx/access.log  main;

    sendfile            on;
    tcp_nopush          on;
    tcp_nodelay         on;
    keepalive_timeout   65;
    types_hash_max_size 4096;

    include             /etc/nginx/mime.types;
    default_type        application/octet-stream;

    include /etc/nginx/conf.d/*.conf;
}

conf.d/proxy.conf:

server {
    listen 80;
    server_name www.example.com;
    location / {
        proxy_pass http://10.0.20.1;
    }
}