"Error 521: Web server is down", but everything is normal

I have received a mail from cloudflare stating “Error 521: Web server is down”, but everything is normal on server.
I checked the logs and firewall rules as per suggestion. There were no error log in server, also the firewall rules allow all IPs for https. May I know what else could be the reason?
After this mail, I have added cloudflare IP ranges in firewall rules as stated here https://www.cloudflare.com/en-gb/ips/, although I suppose that should not be required as per earlier IP 0.0.0.0/0. Should I also add [0.0.0.0/0] in firewall configuration? Also, does cloudflare you inform about IP range changes or is there any place to subscribe to have email alert on cloudflare IP range changes?

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