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=“” Report-To: {“endpoints”:{“url”:“["}],“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?