Thank you for answering so quickly.
The website is connected to Cloudflare through the host (vimexx.nl).
What exactly do you mean with ‘your domain is not on cloudflare?’
This is not the case. I have successfully used various combinations of Varnish Cache (Open Source) and Varnish Cache Plus (Paid) with TLS termination using Nginx, Hitch and Varnish In-process TLS.
The 503 is coming directly from Varnish, so that is the place to direct your investigation.
I am afraid questions regarding Varnish are better asked at StackExchange or any other Varnish related forum. But generally speaking there is no issue in the context of Cloudflare.
As for Cloudflare, your domain is not configured for Cloudflare in the first place, hence that won’t work. You first need to complete the setup.
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.
You may be using a Cloudflare plugin, but your site is most definitely not using Cloudflare DNS or Proxy services. I’m about to close this thread, as this not a general server support forum.
The error you’re getting is caused by the fact that Varnish cannot reach your origin server. This means that the original webserver that produces the content could not be reached.
Varnish offers tooling to figure out what caused this. The varnishlog program allows you to access the memory logs that Varnish produces
By running varnishlog -g request -q "ReqUrl eq '/'", you can filter logs for the homepage. The output will be quite verbose, but will contain all the information you need.
A hypothetical log line that might explain what’s going on, is the following:
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.