Error 521 only for few seconds

Hi there,

I have been experiencing occasional 521 errors on our web app, which resolves itself after few seconds. It does appear quite often (randomly when working in Wordpress administration - 5 times in an hour). I get the error in backend as well as on the production front end.

There are no blocking rules in htaccess file and I have checked all IPv4/IPv6 ranges are allowlisted in IPtables on the server side. There are no other security measures in place regardning this.

I have SSL set to Full (strict) on Cloudflare, using Let’s Encrypt certificate. There was no issue before, I started to notice it few weeks ago.

If I stop using Cloudflare, there is no issue at all.

I run the web app on Wordpress / Woocommerce on dedicated HW server with Debian. The domain is https://koreanconcept.cz

Have you see such issue?
Thank you!

I don’t know why I’m blocking the site

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