I have few Websites running Varnish, but have seen either a combination where users would have Nginx and over it they would install Varnish (not needed) and over that all put Cloudflare. Therefore they have had some issues regarding the cache settings.
In this case a backend named default in the VCL configuration named boot was not reachable. Your VCL file will contain host and port information about that backend.
In your case, the log output might be completely different. Just remember that varnishlog is your friend here, and that a Error 503 Backend fetch failed is not Varnish that is failing: it’s your origin server that is failing.