Subdomain not working (With different server)

I added a site to Cloudflare. This site got 6 subdomains, out of which 5 are hosted at same server as that of the main domain, but a single subdomain is hosted at other server

So for that subdomain, I added an A record pointing to the IP address of that particular server.

Now my main domain and other 5 subdomains are working, but this subdomain which is hosted at some other server is not working.

Cloudflare is returning Error 521, Web server is down while the server is working fine (I have checked properly with both Telnet command. If I pause Cloudflare, sub domains starts working well.

Any help regarding same will be appreciated.

Check If there are Firewall Rules in place that block traffic between Cloudflare and your server. Thats the most common reason. If you have shell access try to ping or trace some Cloudflare IPs

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