I found 3 related topics here, but none of them seem to help me. I have a 4GB Ram server on Vultr and I get a lot of 503 Backend fetch failed on application errors. It’s all over my applications. Not just one. So it has to be a server issue. Traffic is not a problem. I opened a ticket, but until now they haven’t been able to fix it. Any idea if I can try anything myself?
I am getting the same kind of errors on my Google instance. I use sucuri for my dns/waf. Try disabling varnish to see if that makes a difference.
For me similar problem was resolved by disabling Varnish.