Error 503 Backend fetch failed

Let's start, the Varnish it is a "accelerator" , it is a page caching system, it is a reverse proxy, that is, requests go through it and copies are stored in memory.

Usually this error occurs when it exceeds the storage limit in memory, you can edit the file the file varnish (or varnish.params) and adjust parameter http_resp_hdr_len so that the server can support higher consumption (it also depends on how much memory your server has available for the service, if it is Shared it is likely to be quite limited).

Configuration files in different distros:

• CentOS 6: /etc/sysconfig/varnish
• CentOS 7: /etc/varnish/varnish.params
• Ubuntu: /etc/default/varnish

However if you do not have access to the settings only by contacting the hosting.

I'll make one thing very clear , I'm not defending the hosting most have small problems even that often cause inconvenience, but this problem most of the time is the fault of web-developers and site administrators (in the case probably you), who inject numerous javascript, css, plugins and add-ons without need and with this they are making the site More and more heavy and slow, even with incredible caching systems and etc.the server can not stand.

I.e. it is not quite a configuration problem, but rather a problem of misuse of resources, so you can try to give a general review, analyze if everything is necessary or even think about exchanging Wordpress for a more efficient platform, outside this does not have much to do.