nginx empty response after reboot

I bought a new VPS, installed Ubuntu server 20.04, nginx,php7.4-fpm,MariaDB via apt.
I scp uploaded my (working on my Ubuntu 20.04 laptop) PHP project. I could see the 'thank you for using nginx' page,both via ssh curl and through browser from my laptop, but when I tried to use dm.mydomain.tld, both returned noting. The request was logged in access.log
The ownership of /var/www/dm is root, same as /var/www/html. UFW disabled, no SELINUX.
The dm.conf is as follows:

server {
    listen 443 ssl; #http2 ;
    listen [::]:443 ssl; #http2;

    root /var/www/dm;
        ssl_certificate /etc/ssl/certs/cloudflaresource.pem;
        ssl_certificate_key /etc/ssl/private/cloudflaresource.key;

    # Add index.php to the list if you are using PHP
    index index.php; # index.html index.htm index.nginx-debian.html;

    server_name dm.mydomain.tld;
        autoindex on;
    location / {
        # First attempt to serve request as file, then
        # as directory, then fall back to displaying a 404.
        try_files $uri $uri/ =404;
    }
    location ~ \.php$ {
        fastcgi_pass unix:/var/run/php/php7.4-fpm.sock;
    }
}

curl output from ssh console:

root@VPS:~# curl -vk https://dm.mydomain.tld
*   Trying 127.0.0.1:443...
* TCP_NODELAY set
* Connected to dm.mydomain.tld (127.0.0.1) port 443 (#0)
* ALPN, offering h2
* ALPN, offering http/1.1
* successfully set certificate verify locations:
*   CAfile: /etc/ssl/certs/ca-certificates.crt
  CApath: /etc/ssl/certs
* TLSv1.3 (OUT), TLS handshake, Client hello (1):
* TLSv1.3 (IN), TLS handshake, Server hello (2):
* TLSv1.3 (IN), TLS handshake, Encrypted Extensions (8):
* TLSv1.3 (IN), TLS handshake, Certificate (11):
* TLSv1.3 (IN), TLS handshake, CERT verify (15):
* TLSv1.3 (IN), TLS handshake, Finished (20):
* TLSv1.3 (OUT), TLS change cipher, Change cipher spec (1):
* TLSv1.3 (OUT), TLS handshake, Finished (20):
* SSL connection using TLSv1.3 / TLS_AES_256_GCM_SHA384
* ALPN, server accepted to use h2
* Server certificate:
*  subject: O=CloudFlare, Inc.; OU=CloudFlare Origin CA; CN=CloudFlare Origin Certificate
*  start date: Mar  5 07:31:00 2022 GMT
*  expire date: Mar  1 07:31:00 2037 GMT
*  issuer: C=US; O=CloudFlare, Inc.; OU=CloudFlare Origin SSL Certificate Authority; L=San Francisco; ST=California
*  SSL certificate verify result: unable to get local issuer certificate (20), continuing anyway.
* Using HTTP2, server supports multi-use
* Connection state changed (HTTP/2 confirmed)
* Copying HTTP/2 data in stream buffer to connection buffer after upgrade: len=0
* Using Stream ID: 1 (easy handle 0x561a6f79e880)
> GET / HTTP/2
> Host: dm.mydomain.tld
> user-agent: curl/7.68.0
> accept: */*
> 
* TLSv1.3 (IN), TLS handshake, Newsession Ticket (4):
* TLSv1.3 (IN), TLS handshake, Newsession Ticket (4):
* old SSL session ID is stale, removing
* Connection state changed (MAX_CONCURRENT_STREAMS == 128)!
< HTTP/2 200 
< server: nginx/1.18.0 (Ubuntu)
< date: Sat, 05 Mar 2022 13:58:11 GMT
< content-type: text/html; charset=UTF-8
< 
* Connection #0 to host dm.mydomain.tld left intact

you can see that the response is empty.

Additionally, this is the access.log entry.

127.0.0.1 - - [05/Mar/2022:15:44:13 +0100] "GET / HTTP/2.0" 200 0 "-" "curl/7.68.0"

UPDATE: I created an html file and curl it, it worked fine. But for a simple php helloworkd, it doesn't work.

Since the problem aroused after reboot, so I checked the unix domain socket. The ownership of /var/run/php/php7.4-fpm.sock (run ll in /var/run/php/) was www-data, which shouldn't be as I didn't change that.
So I guessed that it was created by nginx, as there's none. By simply change the entry in dm.conf, everything worked:

/var/run/php/php-fpm.sock

Also, remember to uncomment this line:

include snippets/fastcgi-php.conf;

What bothered me was that there's no error reported.