Notification about Error 521 without proxy

[vaz.luiz]

What is the name of the domain?

vkcsubway.com

What is the error number?

521

What is the error message?

There has been a spike in 521 errors from your origin

What is the issue you’re encountering

I received this notification on my email, but I can’t find anything about that on the dashboard

What steps have you taken to resolve the issue?

• Check if any Cloudflare range is blocked on my server firewall: OK
• Check web server logs about any related issue: OK
• Check if the website is working correctly: OK

What are the steps to reproduce the issue?

I can’t reproduce

Screenshot of the error

[vaz.luiz]

To clarify, I am using Cloudflare without the proxy feature.
The domain record points directly to the web server IP.

I don’t understand why I am receiving this notification.
Thanks