Error 503 Backend fetch failed

Let's get started, Varnish is an "accelerator" , it is a caching system for pages, is a reverse proxy , that is, the requests pass through it and copies are stored in memory.

Generally, this error occurs when you exceed the storage limit in memory, you can edit the file varnish (or varnish.params ) and set http_resp_hdr_len parameter for the server to achieve (it also depends on how much your server has available memory for the service, if Shared is likely to be very 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 just by contacting the hosting.

I'm going to make it very clear , I'm not advocating hosting, most have small problems even though they often cause inconvenience, but this problem is most often is the web-developers and site administrators (in this case probably you), who inject countless javascript, css, plugins and needlessly and thus make the site increasingly heavier and slower , even with incredible caching systems and etc. the server has no way to handle it.

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