What is the curl error 52 “empty reply from server”?

This can happen if curl is asked to do plain HTTP on a server that does HTTPS.

Example:

$ curl http://google.com:443
curl: (52) Empty reply from server

Curl gives this error when there is no reply from a server, since it is an error for HTTP not to respond anything to a request.

I suspect the problem you have is that there is some piece of network infrastructure, like a firewall or a proxy, between you and the host in question. Getting this to work, therefore, will require you to discuss the issue with the people responsible for that hardware.

It can happen when server does not respond due to 100% CPU or Memory utilization.

I got this error when I was trying to access sonarqube API and the server was not responding due to full memory utilization