Error 503 Backend fetch failed

Here is the difficulty:

[raw] => HTTP/1.1 503 Service Unavailable Date: Fri, 13 May 2022 11:43:52 GMT Content-Type: text/html; charset=utf-8 Transfer-Encoding: chunked Connection: close retry-after: 5 age: 0 via: 1.1 varnish (Varnish/6.5) x-varnish-cache: MISS x-varnish: MISS x-powered-by: Fastest Cache CF-Cache-Status: DYNAMIC Expect-CT: max-age=604800, report-uri=“https://report-uri.cloudflare.com/cdn-cgi/beacon/expect-ct” Report-To: {“endpoints”:{“url”:“[https://a.nel.cloudflare.com/report/v3?s=2GcJmXmFGa9eVokinrhh4NMvyszF8sv9TisJSIfrwIRAqnfUPPGk6rQ8uUB10q89%2BVowTwspYCaxDw1syahrqo2HFZYYYdbLaeo1KZDu66IBY9umh6al6bdS%2F1pe8xGs2g%3D%3D"}],“group”:“cf-nel”,"max_age”:604800} NEL: {“success_fraction”:0,“report_to”:“cf-nel”,“max_age”:604800} Server: Cloudflare CF-RAY: 70ab238fdc5b99bc-CDG alt-svc: h3=":443"; ma=86400, h3-29=":443"; ma=86400

Error 503 Backend fetch failed
Backend fetch failed
Guru Meditation
XID: 267093948

What to do in this case?