Cloudflare returning 503s immediately on authentic traffic

What is the name of the domain?

What is the error number?

503 Service Unavailable

What is the issue you’re encountering

Cloudflare is suddenly returning 503 server codes (“Service Unavailable”) on a high percentage of visits to our website.

What steps have you taken to resolve the issue?

It seems to impact only logged in users
The 503 is thrown by Cloudflare immediately. It is not a timeout.
We have no rate limiting rules in effect.
We haven’t had an unexpected spike in traffic that would be misinterpreted as a DDoS attack
We haven’t had this issue previously
We are on the Pro Plan.
We checked the server-side error logs. We saw no corresponding errors there.
The 503s are not recorded as Security > Events, no security rules invoked. They are visible under only Security > Analytics
We have page cache enabled, and 503s are returned even on pages that SHOULD be served over cache

Was the site working with SSL prior to adding it to Cloudflare?

Yes

What is the current SSL/TLS setting?

Flexible

What are the steps to reproduce the issue?

Visit pages at forceofnatureclean.com. Sometimes it returns 503s.

Screenshot of the error

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Speed Brain was causing this. When we turned it off there were no more 503s thrown by Cloudflare.