Unexpected end of file” implies that the remote server accepted and closed the connection without sending a response. It’s possible that the remote system is too busy to handle the request, or that there’s a network bug that randomly drops connections.
It’s also possible there is a bug in the server: something in the request causes an internal error, and the server simply closes the connection instead of sending a HTTP error response like it should. Several people suggest this is caused by missing headers or invalid header values in the request.
With the information available it’s impossible to say what’s going wrong. If you have access to the servers in question you can use packet sniffing tools to find what exactly is sent and received, and look at logs to of the server process to see if there are any error messages.