Error 521 - web server is suddenly down


I am using a flexi SSL certificate to pass through traffic from http to https. It worked on 4 of my sites, but now one of them is suddenly failing - but the origin is clearly accessible.
Cloudflare reports err 521, but it seems to be incorrect. I have turned off the force SSL option to check:

With cloudflare flexi SSL enabled:

Clearly, the non-encrypted origin is accessible, but for some reason cloudflare can’t connect. It worked up to this point. Nothing was changed on the server as far as I know, it just suddenly stopeed.

Is this an issue with cloudflare?

Thank you!

I’d check rate limiting with your host, other ideas here if you keep hitting this issue, Community Tip - Fixing Error 521: Web server is down