Occasional 521 errors

Hi everyone! Having a bit of an unusual issue here; hoping someone will have some feedback.

By and large, my website is running smoothly with no issues or server errors. However, occasionally (around 1-2% of the time, per the error analytics) I will get a 521 error. It lingers for a couple of minutes, goes away, and then will come back at some period of time later. There’s no real anticipating when it happens or how.

I’ve already disconnected CloudFlare from the site and in doing so, the errors went away – hence, it not being a direct issue with the origin server. Yet, I’d like to keep using CloudFlare. I’ve tried many solutions suggested, but they seem to be more geared towards consistent 521s as opposed to one that just pops up here and there.

Thanks in advance for any help!

Error 521: web server is down

Error 521 occurs when the origin web server refuses connections from Cloudflare. Security solutions at your origin may block legitimate connections from certain Cloudflare IP addresses.

The two most common causes of 521 errors are:

  • Offlined origin web server application
  • Blocked Cloudflare requests

Resolution

Contact your site administrator or hosting provider to eliminate these common causes:

  • Ensure your origin web server is responsive
  • Review origin web server error logs to identify web server application crashes or outages.
  • Confirm Cloudflare IP addresses are not blocked or rate limited
  • Allow all Cloudflare IP ranges in your origin web server’s firewall or other security software
  • Confirm that — if you have your SSL/TLS mode set to Full or Full (Strict ) — you have installed a Cloudflare Origin Certificate
  • Find additional troubleshooting information on the Cloudflare Community.

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