ssh_exchange_identification: Connection closed by remote host under Git bash

I just ran into this today and it was because the server I was trying to connect to was overloaded with processing. So it may be possible that the server is low on memory or CPU starved.

For fixing the issues add the Hostname for Git on ~/.ssh/config,

Host github.com
 Hostname ssh.github.com
 Port 443

In my case github!

I had this problem today and I realize that I was connected to 2 differente networks (LAN and WLAN), I solved it just disconnecting the cable from my Ethernet adapter. I suppose that the problem is caused because the ssh key is tied with the MAC address of my wireless adapter. I hope this helps you.